For a long time it had looked as though this could be added to the considerable list of indignities engulfing Rafael Benítez. It is not often the chants of "sacked in the morning" emanate from both sets of supporters and, at 2-0, it was shaping up to be the kind of result to accelerate the process of changing that word he dislikes so much. "Interim" might conceivably have become "former" if Chelsea had finished this match as they started.
What happened instead can be partly attributed to the sapping effects a traumatic defeat to Real Madrid had had on Sir Alex Ferguson's players. They looked weary, mentally as well as physically, and were fading badly by the end. Yet those excuses only stretch so far and this still represents a victory of sorts for Benítez. There were cries of "You don't know what you're doing" from Chelsea's supporters when he took off Frank Lampard and Victor Moses early in the second half. He should cherish what followed because these kind of moments have been all too rare in his brief, often tumultuous spell in charge.
It was the notification that this Chelsea team, for all their problems, still have the capacity to trouble accomplished opponents and their badly beleaguered manager does, perhaps, know a thing or two more than the club's mutinous supporters might want to admit. The first goal came from one of his substitutes, Eden Hazard, and was a beauty. The second, from Ramires, followed a wonderful move of classy, incisive counterattacking. Benítez's team have not been renowned for their perseverance and competitive courage under his watch but they would have booked a place at Wembley were it not for an exceptional save from David de Gea, jutting out his right boot to deny Juan Mata in the last minute of normal time. Even then, there were still three separate occasions when Chelsea's adventure and penetration might have won the match and prevented the rigmarole of trying to shoehorn a replay into an already congested fixture schedule.
The transformation was remarkable bearing in mind the way they began the match, riddled with errors, looking short of confidence and perhaps suffering their own fatigue. Ferguson was entitled to blame tired legs and minds but Chelsea, lest it be forgotten, did not get back from their Europa League tie against Steaua Bucharest until the early hours of Friday.
They had looked like obliging opponents at first for a United side trying to shake the Madrid defeat out of their system. The home side's goals both arrived inside the first 11 minutes, first from Javier Hernández and then Wayne Rooney, and Chelsea's defending was so erratic in the first half the damage could have been even more substantial.
It began five minutes in when Hernández peeled away from Gary Cahill and applied just the precise amount of elevation to Michael Carrick's perfectly weighted ball. Carrick's vision and technique, from fully 40 yards, made long-ball football look beautiful but the goal was a wretched moment for Petr Cech, charging off his line and stranded in no man's land as Hernández's twisting, improvised header looped over him.
Rooney, restored to the starting lineup, doubled the lead when a free-kick intended as a cross eluded everyone before bouncing sharply off the turf to deceive Cech and, at that stage, Chelsea were little short of a mess at the back. David Luiz and Demba Ba had both jumped to head the ball and missed. Before half-time Cech had prevented David Luiz scoring an own goal, straight after denying Rooney a second.
As for Chelsea's best opportunity, Moses's effort was so wild it connected with the corner flag. These were moments when Benítez must have felt very lonely as Old Trafford rejoiced in the difficulties for a former Liverpool manager and Chelsea's supporters turned their hostility on him.
He deserves credit because Hazard's introduction played a considerable part in the turnaround. Mata gradually emerged as the outstanding player on the pitch. Cahill had been just as vulnerable as David Luiz in the first half. Yet Chelsea's defenders shook their heads clear after the break and, suddenly, it was the players in red making unforced errors.
By the end, even a player as refined as Carrick was misplacing passes. Rafael da Silva had regressed to the impetuous, raw full-back who can endanger his own team. Patrice Evra was not a great deal better on the opposite side. Ferguson concluded that both his full-backs were "knackered". Fortunately for United, Rio Ferdinand played as though determined to impress the watching Roy Hodgson. His clash with Fernando Torres, a late substitute, aggrieved Chelsea and probably smacked of his own frustrations.
Hazard's goal came seven minutes after replacing Moses, running through the left-hand channel, showing the ball to Da Silva and then curling a right-foot effort into the opposite corner.
After that, Chelsea often pinned back their hosts with the speed and ambition at which they counterattacked. Their equaliser was classy in its creation, with Ba and Oscar both involved as they swept upfield, culminating in a left-foot shot from Ramires that beat De Gea despite the Spaniard getting his fingertips to the ball. Ferguson brought Robin van Persie off the bench and moved Rooney to the left but the tactical change had little effect. Nani had left with a hamstring injury and his replacement, Antonio Valencia, is having an undistinguished season. United, as Ferguson admitted afterwards, were lucky not to be out.