So the shock was there as the old bitter truths of football resurfaced , on a suddenly autumnal afternoon, with all their raw imperatives. Every football man knows the way it goes - the more you win, the more you need to win and heaven help you, whoever you are, when you stop doing it.
Bill Shankly, the founder of Liverpool's dynasty, died of a broken heart. Jock Stein was offered a job in the Celtic pools office not so long after winning the European Cup. Don Revie put on a false beard as he hugged the shadows after tasting failure with England.
Now Ferguson, the man who so brilliantly developed United's status as the most charismatic sports franchise in the world, had the boos in his ears. Football, yet again, had turned a full, relentless circle.
No one in contemporary football has lived so long - Ferguson is in his 20th season at Old Trafford - with these realities more knowingly than the man who grew up in the heart of the Govan shipyards.
But it was not the scale of the protest against the only manager ever able to take hold of the dreams of Sir Matt Busby without being diminished that so invaded the senses as Blackburn Rovers, a team recently pilloried as the archetypal representatives of Doomsday football, won 2-1.
Yes, there were chants of "4-4-2", a rebuke to Ferguson's decision to persist with a 4-5-1 formation, and exclude Wayne Rooney, and then at the end the manager was accompanied by lower-to-middle range boos when he walked down the tunnel. No, it was not football's equivalent of the storming of the Bastille, but a stark possibility had been created: it might just have been the start of the end of something, the end of the astonishingly enduring aura imposed by the greatest manager of his age.
There seemed, however, to be one certainty among the growing doubts intensified by the news Chelsea had recovered from a one-goal deficit against Aston Villa and put another notch on their dismayingly profound control of the early going in the Premiership race, leading second-placed Charlton by six points and their principal rivals, United and Arsenal by, respectively, 10 and 11 points.
The extreme likelihood was that Ferguson, who at 63 has had so many opportunities to walk away from football with an unassailable reputation, would not waver at the first concrete signs of dwindling prestige among the people for whom he had occupied God-like status for so long. A turbulent, erratic, ferocious, angst-filled god, maybe, but a god none the less and one who would always find a way to win.
From Ferguson there was an admission of concern at Chelsea's progress, and regret that his team had missed so many chances against Blackburn, but, typically, no acknowledgement that the dogs of recrimination were now running and barking on the terraces of Old Trafford.
Reaction to the boos was left to his conqueror, and former United hero, Mark Hughes, who said: "He will respond as he always does ... with more determination and more desire to get it right. It's not for me to comment on his decision to leave out Wayne Rooney. I don't comment on other managers' selections, least of all Sir Alex's because he knows a hell of a lot more than I do. But they are all quality players when you come up against any United side."
Ferguson explained that he left Rooney out because he was suspended from tomorrow's Champions' League game with Benfica, saying, "We would have preferred to play Wayne but you have to be fair to other members of the squad who have to replace him. That is the reason I left him out.
"We know exactly what the problem was. We had lots of chances and we didn't take them - we can have no complaints. It was a big disappointment but the biggest disappointment was our defending."
For Ferguson's admirers there is now a jumble of regret and a few lingering hopes. The major fear is that there may no longer be a relatively painless point of abdication from his magnificent reign at United, as there was in 1999 when he won the historic hat-trick of Champions' League, Premiership and FA Cup and finally stood beside his legendary predecessor Busby; in 2003, when he rallied his team from the dead to deny Arsenal the Premiership title, and this last summer when the takeover by the Glazer family promised days of turmoil and changing priorities.
But then Ferguson had already stepped back from the possibility of self-imposed exile. He had looked into the prospect of retirement from the adrenalin surges of football and decided he would be housing his deepest instincts, and satisfactions, in nothing so much as a void.
He had made his bed and if the clothes were a little rumpled on Saturday, if the sounds from the street were discordant, it was still true that the new week would bring United's classic opponents Benfica - and another chance to show that he had still had the heart, and the stamina, of a winner.
Beside the regrets of Ferguson's friends, there is the hope that he will rekindle some of his more naturally aggressive instincts about how to play the game. While it is true some decent marksmanship, ironically from such as Ruud van Nistelrooy and the beloved Paul Scholes, and defending somewhat closer to adequacy, would probably have delivered victory, and quietened the terraces, there is a growing belief that Ferguson's Portuguese assistant Carlos Queiroz has been too successful in his counselling of caution.
And the result? A United team that, while suffering from the loss of authority made inevitable by the ageing of Roy Keane, seems to have lost belief in its inalienable right to victory. That, beyond any tactical wisdom, has always been Sir Alex Ferguson's greatest gift to Manchester United. Maybe never before has it been so desperately needed, by his team - and, the sound of Old Trafford says - by himself.