“Ah, but I was so much older then, I’m younger than that now” — Bob Dylan

The title of the song, My Back Pages, might have been chosen for Sir Alex Ferguson, but in 1964, when fearless Fergie was forsaking the toolmaker’s trade for a full-time contract with Dunfermline Athletic and a resolution to elbow his way to the forefront of the Scottish game, Bob Dylan had passions other than football on his mind.

It is doubtful that young Ferguson took much notice of the poet. His musical taste was oddly mainstream, according to Gordon Strachan, who encountered him at Aberdeen 14 years later and remembers tapes of “some awful Glaswegian who sang in pubs” being played in his car when manager and player travelled to watch matches together. But Dylan’s words would certainly have a resonance for Ferguson now.

He was so much older when Chelsea went to Old Trafford on the evening of May 10, 2005. Their manager was José Mourinho and they had just won the first of two league titles under him. Ferguson ordered his Manchester United players to applaud them on to the field. Some, including Roy Keane, affected sincerity. Gary Neville, of course, did his Kevin The Teenager act.

It was United’s last home match of the season and Chelsea wiped the floor with them. There was an especially beautiful performance from Eidur Gudjohnsen, and Tiago and Joe Cole also scored in a 3-1 win. Thousands of United supporters could not bear to wait for the final whistle and, when Ferguson joined his players on the pitch for a trudge of thanks, the stadium was half-empty.

Ferguson, moreover, was limping. Suddenly he appeared gaunt and almost frail. He was 18 months short of senior-citizen status and beginning to look the part.

Symbolism reared its ugly head. For United were to finish third for the second year in succession. They were to trail in behind Arsenal as well as — by no fewer than 18 points — Chelsea. They had been beaten home and away by AC Milan in the first knockout round of the Champions League and, though we did not know it at the time, were about to be denied the consolation of an FA Cup triumph when Arsenal won on penalties at the Millennium Stadium.

No longer was Ferguson the man for all seasons and the emergence of Mourinho, two decades his junior and with the vast riches of Roman Abramovich at his command, seemed to confirm that time and tide had beaten him.

He’s younger than that now. United have since won three domestic championships and a Champions League. Chelsea were to win only one more championship under Mourinho and get no nearer Champions League glory than defeat on penalties by United in 2008.

Since May 2005 they have exceeded United only in the number of managers engaged. Chelsea have hired four — Avram Grant, Luiz Felipe Scolari, Guus Hiddink and Carlo Ancelotti — and United required none but Ferguson.

This is the alternative context in which we should see tomorrow’s match at Stamford Bridge. Yes, United have a lot to prove. Yes, they did go to Liverpool two weekends ago for arguably their biggest match since the flop against Barcelona in Rome five months earlier — and flopped again. Yes, they have defensive problems and a midfield that hardly compares to Chelsea’s. Yes, it is difficult to recall an encounter with a fellow member of the top-four society into which they went as such underdogs.

And, yes, if they lose, they will be five points behind the best Chelsea team since Mourinho’s peak.

But United have a substance that protects them at times of crisis. It is called deoxyribonucleic acid and Ferguson referred to it by its more common name, DNA, after his team had fought back from 3-1 down against CSKA Moscow on Tuesday to earn the point that secured a place in the Champions League’s last 16. Ferguson is not wholly responsible for that, but his achievement is to have revived and enhanced what you might call the Spirit of Busby.

Sir Matt Busby, his distinguished precursor, survived the Munich air crash and rebuilt. United’s great fortune was to have identified, through the acumen of Sir Bobby Charlton and others, one of the few men who could have invested the Busby legacy and made a profit.

Every time he brings in a young player, Ferguson measures him against the task. Cristiano Ronaldo more than measured up. So has Wayne Rooney. Darren Fletcher epitomises Ferguson’s gift for nurture. Others will be assessed tomorrow — are they “United players”? — and they know it.

No wonder the United board members dread the day when Ferguson comes to them and says it is time for a new and, perhaps, less taxing challenge. No one practises rejuvenation like him. So do not mistake even a heavy defeat tomorrow for the beginning of the end.