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