There will be plenty who hark back to yesterday's extraordinary events in east Manchester and burble on about the magic of the FA Cup but they are missing the point. This was about more than the FA Cup; this was about what has become the most compelling current rivalry in English football. This was a battle to the very end, when, with hands on one another's throats, the two great teams of Manchester had all but forgotten what competition they were playing in.
It could have been the Champions League – or rather, the Europa League – and it could have been the Manchester Senior Cup, what was important was the spectacle these two sides produced in their battle for supremacy.
What a strange outcome, when Manchester United run out the winners, having been three goals ahead at half-time and Sir Alex Ferguson berates them for their carelessness in almost losing this FA Cup third-round tie. By contrast, Roberto Mancini, whose side have fallen at the first hurdle in the one competition in which they are the holders, claims the game will be the springboard for his team to win the League.
In the meantime, Vincent Kompany was controversially sent off and, as it was his second dismissal of the season, will be banned for four games. Wayne Rooney returned from the doldrums of his Boxing Day night-out punishment with two goals. Anders Lindegaard did nothing to dispel Ferguson's concerns over his goalkeepers. Owen Hargreaves played for the first time since November. Later United drew Liverpool away in the fourth round.
Did I mention that Paul Scholes also came out of retirement? Just 225 days after what was supposed to be his last game for United, he responded to Ferguson's dire shortage of central midfielders by coming off the bench for 31 minutes and, mercifully, avoided picking up the kind of booking that became a feature of the tail-end of his marvellous career.
That said, Scholes looked exhausted by the end of the match and, given the pace it was played at, and his body's previous expectation that it no longer had to put itself through these ordeals, it was hardly surprising. Scholes was a glorious, talented footballer and his return enraptured United fans but that novelty will not last forever.
It is a curious fact that Scholes is the first new – if new is the word – central midfielder United have signed since 2007. His return was naturally a delight for United fans, even if it was a strange time to bring him on with City having pulled a goal back. As it turned out, it was Scholes who gave the ball away to Pablo Zabaleta, who crossed for Sergio Aguero to score. What United really need is a midfielder of Scholes' calibre – of eight or nine years ago. But then, everyone wants a player like that.
In many respects, this victory obscured United's deep-seated problems. They withstood early pressure, took the lead through Rooney and then, after Kompany was sent off – correctly, given modern criteria – scored two more before half-time. They would have led 4-0 if Danny Welbeck, the scorer of their second, had done better with Antonio Valencia's cross on 45 minutes.
After the break, United deteriorated badly. It was more than the carelessness which Ferguson identified, it was a shaky defence and a tendency to give away fouls, as Patrice Evra did for the free-kick from which Aleksandar Kolarov scored City's first. There was also the issue of Lindegaard, who spilled Aguero's initial shot on 65 minutes which allowed the Argentine to score the rebound.
It would have been a reckless decision to bring David de Gea back in for a game of this magnitude, especially as Lindegaard just about passed muster in the 3-0 defeat to Newcastle on Wednesday. Even so, he has hardly made himself undroppable. Back in central defence, Chris Smalling ensured United were more reliable but Rio Ferdinand's form continues to raise questions. Phil Jones was fortunate to get away with a handball with eight minutes left.
For all that, there was some brilliance about United, such as Rooney's header for the first goal, which he had set in motion with a ball to Valencia to cross. Welbeck also looked good and seized on his chance brilliantly, after Nigel de Jong hesitated fatefully when he might have cleared the ball after half an hour.
Welbeck also won the penalty from which Rooney scored at the second attempt. Costel Pantilimon, in for the rested Joe Hart, saved the initial shot but Rooney headed in the rebound. There was an issue over whether he raised two fingers to the City fans in response or simply cupped his ear. Earlier on, he could certainly have done without gesturing that referee Chris Foy should send off Kompany.
That was for a foolish tackle by the City captain on 12 minutes. Although he made minimal contact with Nani, that aspect was, as former referee Dermot Gallagher later said, irrelevant. A bad injury was not required to prove the tackle was dangerous. Mancini said the club would appeal the red card but they may have a rethink. There is no chance it will be rescinded.
With Kolo Touré now at the African Cup of Nations and Kompany potentially suspended for both Carling Cup semi-final legs against Liverpool, as well as the League games against Wigan and Spurs, this was not quite the triumph in defeat Mancini proclaimed. City looked good in the second half, for which Mancini substituted David Silva and played a 3-4-2 formation that was surprisingly effective on the counter-attack.
Mancini was in sufficiently good humour afterward that, when asked what his reaction to seeing Scholes' on the team-sheet had been, he replied by saying he had told his assistant David Platt that he too was on the bench. That moment was bettered in the first half when Ferguson, furious at Nani being penalised for a foul throw, strode to the front of his technical area to mime how to take one correctly.
These are the two teams who – regardless of the competition – produce compelling games. Like all great rivalries, they are building a rich history, although the next instalment, notwithstanding the potential for the Europa League to throw them together, at the Etihad on 28 April promises to be even more highly charged than yesterday's.
Vincent Kompany's red card rules him out of City's next four games:
Wed LC Liverpool (h)
16 Jan PL Wigan (a)
22 Jan PL Tottenham (h)
25 Jan LC Liverpool (a)
Manchester City: PANTILIMON 6/10, RICHARDS 6, KOMPANY 3, LESCOTT 5, DEJONG 6, NASRI 5, KOLAROV 7, MILNER 7, A JOHNSON 4, AGUERO 7, SILVA 6
Manchester United: VALENCIA 6, NANI 6, ROONEY 8, EVRA 7, FERDINAND 5 SMALLING 7, JONES 6, WELBECK 8, CARRICK 7, GIGGS 7, LINDEGAARD 4
Goals: Kolarov 48, Aguero 64. Rooney 10, 39, Welbeck 31
Substitutes: Manchester City Zabaleta 7 (A Johnson, h-t), Savic 6 (Silva, h-t), Hargreaves (Nasri, 82). Manchester United Scholes 5 (Nani, 59), Anderson (Welbeck, 75).
Booked: Man United Welbeck, Nani, Evra, Ferdinand.
Sent-off: Man City Kompany (12).
Man of the match Rooney. Match rating 9/10.
Possession: Man City 50% Man United 50%.
Attempts on target: Man City 6 Man United 6.
Referee C Foy (Merseyside). Attendance 46,808.