The tie was United's, the Cup removed from City. The momentum, though? That may still be with Mancini.
Ferguson lands blow in latest round of Manchester derby with Mancini
Short on numbers, long on question marks, Sir Alex Ferguson still stopped the bleeding.
Near a team's worth of absentees? The state of his relationship with Wayne Rooney? His necessity to transfer-market spend Manchester United back to health? All were laid aside to deliver a result that kept Manchester City from sprinting away with the domestic season.
Rooney scored twice - and Paul Scholes was seduced back from retirement to a substitute's role - as United exploited a questionable sending off to stream to a three-goal lead. Cleverly reshaped, City cut that back to one in a hectic second half that could easily have ended level. Yet this was not the day for United to lose a third game in a row; nor for City to add back-to-back FA Cup evictions to October's six-goal evisceration.
It was a bizarre afternoon. Ten out of ten in home league games, Roberto Mancini erred in fielding a weakened team in a game of such psychological import then cited the quality of his team's response to claim: "I think now we can win the league."
Ferguson prised the result he needed from a critical fixture, yet bemoaned another example of his team's shortcomings. "I don't think today does us any good at all," he said. "It was a careless performance in the second half. We should have been home and dry. If we keep doing that we're going to make mistakes."
Entering this tie United's on-field issues could be charted in numbers. A pass completion poorer than five other Premier League teams; the average amount of ground covered by players dropping by almost half as the season has progressed.
City's attempts to keep Yaya and Kolo Toure an extra day away from the African Cup of Nations having met unbreakable Ivorian resistance, Mancini had no option but to reshape. Intriguingly, he did it with gusto, treating a tie with United almost as if his third-round opponents came from the Championship. Among three unforced changes, Joe Hart was rested to furnish Costel Pantilimon with just his fourth City appearance.
The 24-year-old Romanian's role was that of a spectator for an opening period in which City ran, passed and harassed aggressively without forcing Anders Lindegaard into a save. Moments, though, alter matches, and two were about to fall United's way.
First, came a compellingly direct attack. Receiving the ball some 40 yards from goal, Rooney immediately redirected it to Antonio Valencia and made for the penalty spot. The winger's cross whipped back towards him at pace and Rooney looped it in off the bar. A first-instinct kiss of the Manchester United badge retorted to reports of his January sale.
Soon Rooney was petitioning Chris Foy, the referee, over an errant tackle. Vincent Kompany seemed to lose the footing of his left boot as he ran in to meet Nani. Whether accidental or intended, the defender ended up tackling with both feet airborne and a red card followed.
"Absolutely" a red card said Ferguson. "I'm sure we will win an appeal," countered Mancini.
United now had control, easily coping with the home attempts to equalise before extending their lead through Danny Welbeck who scored with an over-the-shoulder volley.
Soon it was three after Aleksandar Kolarov felled Welbeck him in the area. Though Pantilimon parried Rooney's spot kick, the save fell generously for the striker to head home.
At the interval Mancini brought on Pablo Zabaleta and Stefan Savic, moved to a three-man defence and asked his team to hold tight and counter-attack. Three minutes into the half Micah Richard's long run won a free kick that Kolarov classily converted.
Ferguson introduced Scholes in central midfield to grand cheers from the visiting support and should have had a second penalty when Kolarov cut down Valencia. Instead, the 10 men were to score again.
A short pass from Scholes allowed James Milner to make ground on the right and centre for Aguero. The striker shot as soon as the ball came to him, forcing an unbalanced Lindegaard to spill it back to him.
Mancini then watched Foy fail to penalise a Phil Jones handball. The tie was United's, the Cup removed from City.
The momentum, though? That may still be with Mancini.