On Sunday, when Manchester United take on Swansea City, a 58-year-old man by the name of Pete Molyneux will lift up an old bed-sheet just as he did, infamously, in December 1989 when Alex Ferguson's back was pressed against a cold, unforgiving wall and the mood inside Old Trafford was of brooding discontent.

The message back then was short and to the point. "Three years of excuses and we're still crap, ta-ra Fergie," the banner read. It has gone down in history, the nadir of Ferguson's reign, and now the man with the paintbrush and the tin of black emulsion plans to makes an updated version. This time it will come, in part, as an apology. Mostly, it will come as a thank you.

Pete, like a lot of United supporters, expects to be holding back the tears during Ferguson's 1,500th – and last – game in charge. His banner will read: "Twenty-three years of silver and we're still top, ta-ra Fergie."

Ta-ra, indeed. Busby closed his reign at Old Trafford when he was 62. Shankly left Liverpool at the age of 60. Paisley lasted at Anfield until he was 64. Clough was 58 when he bowed out. Ferguson is 71, with a pacemaker in his chest and a hip replacement on the calendar, yet the first reaction is still near disbelief and a sense of wonder.

The statue of Sir Alex Ferguson outside Old Trafford. Photograph: Christopher Thomond for the Guardian

The Man Who Can't Retire, one newspaper used to call him. And there he was, after the latest championship, cracking jokes, full of levity and humour, talking effusively about working next season and beyond. Someone asked whether we should take it for granted he would still be in charge and he talked about the "magic pills" still working. He always was particularly skilled at spinning a line, Fergie.

His legacy is as solid as the stadium where his statue – long overcoat, match-face, strict side-parting (it was always a side-parting, even in the 1970s when football went shaggy) – now rises behind the stand they named after him. Thirteen league titles for United, two European Cups, two Intercontinental World Cups, one European Cup-Winners' Cup, five FA Cups, four League Cups. Does the Community Shield count? There were 10 of them.

Add in those years at Aberdeen, when he set about dismantling the Old Firm's dominance, and the trophy count is 49. Manager of the year? Ten times. Manager of the month? Twenty-seven. It's bordering on ridiculous: has there ever been another manager whose silverware has to be totted up like a maths problem?

Nobody has ever done it with so much competitive courage. Nobody has beaten the system like he has. Ferguson has outlasted 24 different managers at Real Madrid, 19 at Internazionale, 18 at Chelsea and 14 at Manchester City. He has seen off prime ministers Thatcher, Major, Blair and Brown and most of us probably expected Cameron would be added to that list rather than joining in the tributes in the House of Commons.

Ferguson's latest title, United's 20th in total, brings him personally level with Arsenal, the third most successful club in the country. It is not strictly true that he knocked Liverpool off their perch but he has certainly done a fine job of helping to make sure they did not clamber back. Liverpool have won the title 18 times. If Ferguson were a younger man – maybe 10 years or so – you would have fancied him to catch and overhaul them, too.

When Ferguson took over on 6 November 1986 United were 19th in the old First Division and had not won the league for nearly two decades. Fuji had just launched the world's first disposable camera. Mike Tyson was a couple of weeks away from his first world title. Nick Berry was at No1 in the charts with Every Loser Wins. It was the year of Crocodile Dundee and Top Gun, Andrew marrying Sarah, Charlene marrying Scott, "Freddie Starr Ate My Hamster" and Wayne Rooney's first birthday.

Ferguson's team for his first game, at Oxford United, was: Turner, Duxbury, McGrath, Moran, Albiston, Hogg, Blackmore, Moses, Stapleton, Davenport, Barnes. Ferguson, determined to get to his first team-talk right, managed to call Peter Davenport "Nigel" by mistake. They lost 2-0.

Twenty-six years on, we probably have a better idea why he could not bring himself to face the media after that defeat to Real Madrid in the Champions League in March and the shattering effects it must have had on him, knowing as he did that it would be his last stab at the trophy he has always coveted the most.

We can imagine how much he will have craved the perfect ending: trophy number 50 at Wembley on 25 May. And it is a strange, almost unnerving feeling knowing that he will not be in the dugout next August. We always knew he was going to have to cut himself free one day but it still comes at you like a mallet. "He's like a seat in the stadium, the grass on the pitch," Roberto Mancini said recently. "He is a part of United."

He is also unique and it is probably only now, and in the future, that his achievements will fully get the recognition they deserve.

At the same time, it is difficult not to think how he will cope without the daily fix. Football has been the thing that has made the most sense in his life and the journalists who have travelled round the world on his coat tails are well accustomed with his usual reaction to any talk of retirement. Apprehension, mostly. At other times, he would treat the question like an affront and, suddenly, you might be in that wind tunnel as he leant in and unleashed all that anger.

But there were other occasions, too, when Ferguson really opened up and the impression he left was of someone who considered life as the former manager of Manchester United might be shapeless and unattractive, scary even. "The big fear is what you would do with yourself," he once said. "There are too many examples of people who retire and are in their box soon after. You're taking away the very thing that makes you alive, that keeps you alive.

"I remember my dad had his 65th birthday and the Fairfields shipyard gave him a dinner in Glasgow with 400 people there. The next week my mother phones and says: 'Your dad's going in for an X-ray, he has pains in his chest.' I said: 'It'll be emotion.' Well, it was cancer. A week. One week."

Ferguson regarded retirement as something, in an ideal world, he could just file away in a drawer.

How will he make his retreat? We know he is staying on as a board member but that 6am routine, into the training ground, a slice of buttered toast and then the "bloody mountain of paperwork" on his desk is going to be a desperately hard routine to shift. More time for the horses, you might think. He has a large, extended family, with a small fleet of grandchildren and no doubt they will all be on the pitch on Sunday when he takes the microphone for the last time. Ferguson always wanted to teach himself to play the piano. He keeps wine and loves to read, always taking in new information. Yet football – or, specifically, football management – is his addiction and it is not going to be an easy habit to kick. Even Ferguson, with that powerful clarity of mind, could be forgiven for experiencing the odd moment of insecurity.

The tributes during the day reflect the sadness that most people feel, because this is not just the retirement of a great manager. It is the departure of the last of his kind: the old-school manager.

No doubt there will always be some who cannot get past the caricature and will not be able to marry all the tributes with their own memories of how Ferguson has behaved to his own rules. Ferguson, let's not sugarcoat it, could be terribly ruthless. He could be cold, vindictive, with little thought about how his actions impinge on others. His press conferences could be tense, joyless affairs. The "hairdryer", that blast of full-on rage whenever he felt affronted, was a remarkable thing to witness and there is no point being hypocritical here: there were moments when he could be, in the vernacular of the business, a proper bastard.

Alternatively, there is a different Ferguson, one that is not seen enough, and maybe we will get to know more about when he is removed from the frontline of his industry.

There were acts of great kindness, for example. Not many people know of the video message Ferguson sent to Paul Hunter, a couple of weeks before his death, telling the former Masters snooker champion he should be proud of everything he has achieved and praising him for his bravery and dignity fighting his cancer. Few column inches were devoted to the fact Ferguson felt compelled to get in touch with the parents of Josh Furber, a student killed on holiday in Australia, after hearing that he was a United supporter. Nor are these isolated cases. Ferguson will go out of his way to attend the funeral of a loyal supporter, an unsung member of staff or one of his many acquaintances.

Yes, there were times when his relationship with the media was fraught and it felt like he was at war with the industry. Yet that has been overplayed, too.

Some of Ferguson's oldest friends have been football writers. He has been known to ring newspaper offices and demand to be put straight through to the editor after hearing that one of the reporters on his patch might be made redundant.

David Meek, the former Manchester Evening News correspondent, always remembers the incredible kindness Ferguson showed him when he was diagnosed with cancer in 2003. Meek's phone rang one day and the message at the other end simply growled: "The Scottish beast is on his way." Ferguson was at Meek's front door 20 minutes later. Meek, who has been Ferguson's ghostwriter of choice for many years, remembers how Ferguson looked him in the eye and told him exactly what he had wanted to hear: "You can handle it."

Even then, there is a delicious irony that Meek put together Ferguson's last set of programme notes for the game against Chelsea without any idea that he, along with everybody else, was being had. The standout line was this: "I certainly don't have any plans at the moment to walk away from what I believe will be something special and worth being around for all to see." Classic Ferguson, right to the end.