Grief is like a furnace: trapped inside it, you become molten, malleable, ready to be fashioned into something different. And sometimes, when everything cools down again, you're stuck in an unfamiliar, awkward new shape, the person you once were fossilised inside. You can see that meeting Lou Macari. The small man who comes bustling towards me in a Manchester hotel shows, on the surface, some of the expected attributes of old: energy, bluntness, common sense. But how quickly you realise what a shell the human body is, just a carapace built of the outer edge of experiences that are carried mainly on the inside. And when you pick that old, crusty shell, no matter how many years it has been forming, fresh blood flows.

Macari has been through a lot in his life. The man who played for Celtic under Jock Stein, for Manchester United under Tommy Docherty, for Ally MacLeod's dream team of the 1978 World Cup, is no stranger to turmoil. Not just losing to Peru, or weathering the heights of ridicule that Ally's Army were exposed to back then, because see that? That was only a game. Macari came to know the difference between life and a game.

Not even being banged up in a police cell, falsely accused of fraud when he was manager of Swindon, registers much. Nor losing all the money he had made as a footballer (loose change to the pampered, David Beckham generation of footballers) when he took Fergus McCann to court after McCann peremptorily sacked him as manager of Celtic. And why is he not bitter about that? Because that's only money. He smiles. Fergus McCann? He wouldn't waste energy thinking about him. "I can laugh now. I can say, 'How big a deal was that? Not very.'"

Strange how familiar Macari seems, how instantly comfortable to be with, despite the conversation being about such troubling things. Because I know his type; we all know his type. That generation of Scottish men who may be a bit buttoned up but so thoroughly decent you'd trust them with your life, and if you ever earn their love or their loyalty then, my God, you're tapping into a deep well. The only drawback being they're more likely to drown in there than tell you.

Macari does not waste words. He has just produced an autobiography, Football, My Life, and right at the start of it he says the events of his life have put football into perspective. Then he barely mentions his private life again until near the end, when in one short section he presents the reader with everything he's going to tell, like he simply boxed it all up and unwrapped it only when forced. It recounts the defining event of his life: the 1999 suicide of his youngest son, Jonathan, when he was just 19.

Was that the only way he could deal with it, to contain it in that way? Macari hesitates. The football side, the facts and the figures, take care of themselves. "You just say what happened. You know you may say one or two things that might upset people, but what's more important is that when you're talking about family, especially someone you've lost… it's difficult, sensitive… the hardest part of the book." It caused a few problems at home, he admits. Maybe that's the way it should be, because when you love someone, everything matters – every word, every thought, every nuance. And talking to Macari, you're in no doubt how loved Jonathan was. How important he was. How different life is now.

When you look at a stopped watch, often the last thing you notice is that it has stopped. It can con you for a while. You see the face, the strap, the hands pointing to a time. It looks normal until suddenly you see that the small seconds hand is motionless. There is about Macari, that sense of apparent normality concealing malfunction. "I've got to put on a false face," he says. If it hadn't happened, he'd be sitting here reflecting on the greatest parts of his life. "Oh, that day at Hampden Park when we beat Rangers and I was a young player and got my first Scottish Cup medal – that was great. And that was followed by a trip to Wembley… and how lucky I've been, I've played for Scotland. But I don't sit down and look back and think what's been great, because one event has washed it all away." It's the same for any parent who loses a child, he says. Parents whose children are killed in Afghanistan or Iraq. You can't ever tell how you'll deal with it. "Certainly, it could stop the rest of your life. You could just come to a complete standstill."

THE ITALIANS WHO emigrated to Scotland often have wonderful stories of how they got here – literally walking from the Italian mountains, across Europe, searching for a new life. Macari has no such story. He talks only reluctantly of his Italian lineage. When Lou was just a year old, his parents moved away from Scotland to London, where they would remain for the next eight years. Lou's father was a grafter, working in the catering industry, while back home his own parents ran a successful café in Largs. There was Nardini's at one end, the Moorings at the other, and Macari's in the middle.

Whatever the reason for Macari's indifference to his Italian roots (I suspect his grandparents would have preferred their son to marry an Italian than a Scot), what's more important is the attitude he displays: a fierce loyalty to his parents, a strong sense of what he believes is right and wrong. He was an only child and his parents died in their 40s. "What they did for me, and how they looked after me… how they got me to Celtic Park when I was starting off as a reserve player… you just don't get the chance to repay them."

His parents had moved back to Largs from London, and Macari's house backed on to the Largs recreation centre where he watched footballers in pre-season training. There he was, 12 years old, wearing his Celtic scarf in the garden and watching Billy McNeill, thinking this was the closest he would ever get to his Lisbon Lions heroes. But at 16 he was signed by Celtic and worked under the legendary Jock Stein, a man who inspired both respect and fear. Stein had them running from Celtic Park to Barrowfield every day and he would drive alongside his players, shouting, "Get a move on!" from the open window.

"I was terrified of him," recalls Macari. "Everyone was terrified of him. You didn't get a second chance with Jock. It's strange to think about it now, but even Coca-Cola was frowned on. If you sneaked a Coke up to your room at night and he spotted it, he'd throw it down the sink and say, in typical Jock fashion, 'I'll Coca-Cola you.'"

That kind of discipline is lacking nowadays. Macari cleaned the Lisbon Lions' boots and dealt with their laundry and felt privileged to do so. There were no guarantees about making it as a player, no fancy wage packets. Now, money has taken the hunger from young players, taken away their sharpness and drive. Macari insists that even footballers regarded as naturally gifted, such as Kenny Dalglish, needed to be pushed. "Was I talented and gifted enough to achieve it on my own?" asks Macari. "No, I wasn't. The man who pushed me along the way, who made me run and made me appreciate everything you get out of the game, was Jock Stein. I was guided in the right direction, given a chance of being a footballer, and that's all it was – a chance."

The Scots in general have a tremendous record in football management. "They have a hunger," explains Macari. Alex Ferguson has a huge start on every other manager because of his disciplined style. "They're all pussyfooting around and players are stepping out of line. Alex is trying his best to keep hold of his beliefs in a changing world, and he probably knows he's not going to achieve that, but I would say he still has a greater hold on his players than any manager in the country. And long may that continue. He worked with Jock and no doubt things rubbed off on him because I see similarities between the two managers. Stein helped me have a long career, and Dalglish and Danny McGrain… all because of the start that was demanded of us, the levels of commitment, of fitness."

It makes him laugh when people talk so earnestly now about football tactics and formations and fancy this and fancy that. He's of the run-your-guts-out-on-the-field school, with fish and chips on the team bus on the way home. It's not foreign players who are keeping the home players out of sides. (He remembers Brazilians being brought by Stein to Celtic and Scandinavians being brought to Morton.) But the home players had the edge because they were driven. He has seen players who didn't really have the natural ability to make it, achieve because they were focused. "So focused they'd stay in at weekends and they'd be the type cleaning their boots in their digs and looking forward to training on Monday morning. Is that happening nowadays? No chance. No chance whatsoever. It's looking forward to the weekend now. I'm a great believer that it's just enough to throw your career off the route it should be going."

He never imagined he would leave Celtic, but he knew he could make three or four times as much in England. "If I had still been the young lad picking up my wages on a Tuesday, still heading into town with Dalglish, McGrain and David Hay, and spending three or four quid on a meal, happy as Larry with no commitments, then fine." But life had changed. He was married, his wife was expecting, and, tragically, his father had died of cancer and he now needed to support his mother. He headed south to sign for Liverpool and ended up being snatched at the last minute by Manchester United, who at that time were far less successful than their Merseyside rivals. He'd have ended his career with more medals at Liverpool but he doesn't regret joining United. He got career decisions right. It's other things in life he got wrong.

In the years following his father's death, his mother stayed in Largs. Macari intended to bring her down to Manchester, and even invested in a chip shop for her to run. But life was busy and the 1978 World Cup was approaching, and she died before he got round to organising her move. He still feels so guilty about that. "You kick yourself for not doing this, not doing that," he says.

He had tried phoning her just three or four days before but had got no answer. Now he was sitting in a hotel with the Scotland team when he looked up and saw his cousin, a doctor, walking through the door. "I thought, 'I haven't seen him for a while,' and by the time he got from the front door to me, I thought, 'Bloody hell, there's something…' His face, the way he greeted me – there was obviously bad news. He'd come to inform me that my mum had been found dead in the house. She had been lying there maybe four or five days and I couldn't go and identify her. That's why he had been called in."

The circumstances of her death were strange, and Macari still has no answers. Does he know why she died? "No," he says. "My mum had been on her own, and in the conversation I'd had with her she said she had some friends up there. Putting the pieces together after she died, I just wasn't convinced that the friends were good friends. Some money had gone missing." She had signed things over to these friends that she shouldn't have. She was on tablets, had taken too many and simply never woke up.

Macari thinks now that she never got over her husband's death. "She never really showed me she couldn't get over the loss of him because it's something a parent would probably want to hide from their child, but I always had a gut feeling that losing my dad would be tough on her, really tough." They were a close couple? "I would have said as close as you get. And I've moved down south and am not really there to gauge how she's coping with it, and of course when I phone up and speak to her she's giving me the impression she's coping fine, which is probably what I would do… what I do at this moment in time with my lad. I've got to be seen to be coping." And is he? He avoids the question. "It's tough, isn't it?" he says instead.

JONATHAN RUNS LIKE a thread through the conversation, a coloured thread against which everything else in life is compared. The talk of discipline among young footballers, of the effect of high wages on their playing, on their psyches… I'm not sure it's just the musings of an older player on the current state of the game. I think maybe we're talking about Jonathan. Macari has three sons, all of whom have played the game professionally. You can tell he's proud of them all. You can also tell he's not a man for making big boasts, but he says Jonathan was really talented, had the ability to go places. He had signed for Nottingham Forest. But things weren't going right.

Did the new football culture he describes contribute to Jonathan's difficulties? "I think it would surprise you, the money," he says. "I argued about it even with my wife. The money he got for walking out of school and into a football club was insignificant to what a lot of them are being paid now, but it was enough to give him a cosy start to his football career and it shouldn't have been. I just felt it was too much." It was life-changing. "If my lad decided to go out every Saturday night because he's got the finance, it can't be doing him any good."

When Macari stopped playing at the age of 34, he had no stash of cash for life. He went into football management, with stints at Stoke, Swindon, Birmingham City and, of course, Celtic, and looks back on the stresses and strains with a kind of indifference. The fraud charges at Swindon came about because the team captain asked him for £40 bonuses for the players. Macari asked the board; they agreed. Macari wasn't to know the payments weren't accounted properly. He had no interest in accounts. The same thing was happening in other clubs and involving far bigger sums, and Macari believes Swindon was used as a warning to higher-profile clubs. But Macari himself was cleared.

As for Celtic, well, Fergus McCann arrived just after Macari was appointed, and Macari felt from the start McCann wanted him out. (In fact, a journalist phoned Macari on McCann's first day to say a senior Celtic figure had told him so.) But it would have cost McCann a lot of money to buy out Macari's contract. Their working relationship was disastrous, with McCann communicating by letter despite their offices being only 50 yards apart. "I didn't bring it to a head because I wanted to be manager of Celtic. That's why I came back, why anybody would have come back. I was willing to put up with all the crap that was going on, willing to put up with him not giving me money to buy players, because I wanted to be manager of Celtic."

McCann knew employment law. Macari didn't. When the chairman eventually sacked him for disobeying an instruction, Macari took him to court. Halfway through, Macari's lawyer was promoted to Lord Advocate with immediate effect. Macari subsequently lost not just his lawyer but the case and his savings. He must have been bitter, surely, losing everything? "At the time you're upset because you know the way it's been manoeuvred and what is being said is untrue. And it leaves a nasty taste in your mouth. Years later, when you see others in the hot seat – Tommy Burns, God rest him, Wim Jansen – it wasn't a personal thing against me. It was the way he was."

It doesn't rankle that he wasn't a successful Celtic manager. "It was never on the cards. It was never to be. People have said Fergus saved Celtic, and I understand why they say it. But if Fergus hadn't come in, someone else would have, because the job was there to do. In fairness to Fergus, that's where he was smart – he realised what Celtic Football Club had going for it, not just at that time but what it has always got going for it: a massive worldwide support."

People generally divide into two camps: those who are embittered by bad experiences and those who shake them off. "I can shake it off," says Macari. "There are other things I will never shake off."

JONATHAN MACARI was small, like his dad, and happy-go-lucky. He was a striker, played up front in a little-and-large combination with Marlon Harewood, a big 6ft-tall centre forward who has gone on to play for Aston Villa. The manager had cleared out a lot of young players but kept Jonathan. He was a prospect. So it was a shock when Jonathan himself decided to leave. Macari didn't understand.

The morning he learned his son was dead, Jonathan's girlfriend had called at the house. Had Jon come back? No, Macari said, he thought he was with her. Jonathan's body was discovered hanging in a wood by a man on his way to work. Macari has never found out what really happened. Jon was closer to his mum, Dale Marie, but she doesn't know either. "There's no point in guessing," says Macari. "I have my own thoughts but it wouldn't be right to put them into print because Jonathan's not here to tell us."

Maybe at 19, life and love are a bit more intense than at other times in your life. There was also talk of drugs but Macari has discounted that now. "At the time, I possibly believed drugs were part of it, but I don't any more. My lad wasn't an angel. He had stepped out of line, but at that particular time he didn't have a problem. The coroner made that clear. If it was drug-related I wouldn't have a problem saying so because it would be giving youngsters an indication of what can happen. But, no, it wasn't the case."

People think of grief as the great unifier, but more often it's the great divider. That's nothing to do with how much you love the people around you, it's to do with everybody expressing their grief differently. I tell Macari I have often been struck by how differently mothers and fathers deal with the loss of a child. A mother once told me she wanted to talk about her dead son all the time. Her husband's way of coping was never to mention him. That difference was hard for both of them.

Macari nods. Later, he says he probably does the opposite of what his wife does. And he has a friend who lost a son and who visits the cemetery every week. Macari couldn't do that. He sees the stone on the next grave, a woman of 89, and it pierces him to see Jonathan's age beside it. But he wouldn't criticise any way of coping with grief. "We all deal with it differently, and I couldn't tell you how the other members of my family are dealing with it because I just don't know."

He has videos of Jonathan playing football. Videos from Sky television of himself being interviewed with his three lads round him. "I could get all those memories back but I'm just not sure I would want to sit in the house and put on a DVD and see that again. I've not asked my wife but she might want that. I don't go down the road of even talking about it."

But he loves his family, keeps going for them. "I just know that I've got a commitment to them and therefore I've got to continue working." He works for Sky and Manchester United TV, and he can't be on screen looking miserable. He has to paint a smile on. But he's not sure he would have the patience or the focus for management any more. Things are different.

When your child dies, you deal with grief and loss. When your child commits suicide, you deal with grief, loss and guilt. "You're asking yourself why, wondering what you could have done." Macari's burden is all too obviously heavy. But if another teenager committed suicide, would he blame their parents? No, he says quietly. Yet he would blame himself for Jonathan's death? "I would, yeah. You take the view, 'Why didn't I do this? Why didn't I do that?' It's difficult to erase those thoughts from your mind. I just felt that I missed things. I wouldn't say they were staring me in the face or I wouldn't have missed them. But why would someone…"

He breaks off. As a manager, he was so careful with youngsters. Tried to protect them. If a talented lad had come in saying he wanted to leave he'd have kept him in his office for a day until he found out why. But he never found out with his own boy. But I'll bet he tried. "I did try, yeah," he says. "I didn't get any answers." Well, how could he possibly have known time was running out to get answers? But nothing will convince Macari not to blame himself. It's why he hasn't bothered with counselling.

Occasionally, interviewing can seem like a worthless kind of dance, chipping at ego, skirting round truth. Today it feels like the most privileged job in the world talking to someone about things that really matter. People say time is a great healer, says Macari. He hasn't found that. No wonder – the phrase implies at some stage you get your old life back. How can you? When you come out of the heat of the furnace that loss imposes on you, you have to take on a different shape. The challenge is to make it a good shape.

But is Macari capable of happiness again? "I couldn't have the streamers out at Christmas, the whoo-hoo… I couldn't. I'd be ashamed of myself, to be honest. It would be a bit of a betrayal. I'm not going to ruin it for other people. I just don't get into that environment. I'll take my kids out and my grandchildren, and hopefully we'll enjoy ourselves. But events like Christmas and birthday parties… I just feel that wouldn't be right."

There is something very, very Scottish about Lou Macari, for all his years in the south. A blunt simplicity. An emotional quality, both diffident and piercing. The poet Hugh MacDiarmid famously wrote about the little white rose of Scotland that "smells sharp and sweet and breaks the heart". A Scottish footballer might be the last person you'd associate with flowers and sentiment. And yet for some subliminal reason, that line returns to me over and over when I leave Macari. Maybe it's because the deepest things in life are often the simplest. Sometimes, when he's on his way home from a good football match, Macari is almost tricked into thinking he's happy, and then it hits him. "You're going back to a house where there's one missing."