This victory will matter to Manchester United regardless of the fact that the FA Cup is the lesser priority. Defeating the holders, Manchester City, after that 6-1 trouncing by them in the league at Old Trafford is to be relished. Sir Alex Ferguson will now expect his men to believe they can compete on equal terms with these opponents in the Premier League.
United were a match for a City side cut to 10 men but it took a while for them to accept that Vincent Kompany was gone. The referee, Chris Foy, had sent him off in the 12th minute after a two-footed lunge although his opponent Nani was not endangered. Officials at large were insistent that Foy's interpretation was consistent with current guidelines.
City will have taken something from a seemingly unequal struggle. Considering they had been 3-0 down to a merciless United – Wayne Rooney opening the scoring two minutes before the dismissal with a superb header from Antonio Valencia's cross – they displayed spirit and, with Roberto Mancini's readjustments, tactical wisdom. In Kompany's absence a 3-5-1 system adopted after half-time had more flexibility than anticipated.
United were troubled by the formation and were also in two minds as to whether they should strive to prey on outnumbered opponents or simply work their way quietly towards full time.
It is to City's credit that they rallied to score twice but their inability to show resilience at the onset came as a surprise. United would have had a better excuse for frailty and anxiety here after the 3-0 collapse at Newcastle United. No one doubts that Kompany was missed or that it is a blow to the club, who will appeal the sending-off. It is most likely they will have to do without their captain for four games since he has already been banned this season, following a red card at Wolves in October.
City seem to rely on Kompany greatly and it is no help either that Kolo Touré has departed with his brother Yaya for the Africa Cup of Nations. United themselves had looked depleted. They are so lacking in numbers that Paul Scholes, now a coach at Old Trafford, was named among the substitutes and will be available for the rest of the season. The 37-year-old was brought on here.
He may have participated in many a win over City and done much to bring about those results but this day should linger in his thoughts since it can have been only wildly unexpected. When Scholes decided his time as a player was at an end last season not many disagreed vehemently. Injuries may have been a handicap to Ferguson but, despite this victory, United's squad still have to be redeveloped.
There should be few grievances to fill the aftermath with quarrels. United sought a second penalty when claiming that Aleksandar Kolarov had felled Valencia but a handball appeal after a shot by the City left-back seemed to come off the arm of Phil Jones was also ignored.
City, much as they will rage against the red card for Kompany, should be embarrassed by their first-half disintegration. United were merciless for a while. Patrice Evra's cross took a deflection but Danny Welbeck was acrobatic enough to volley past Costel Pantilimon. Five minutes from half-time the Romania goalkeeper saved a Rooney penalty awarded for Kolarov's foul on Welbeck but the striker headed home the rebound.
City still took great credit for their second-half endeavours, with Kolarov lifting a free-kick over the wall and past Anders Lindegaard. The struggle grew more intense when Sergio Agüero met James Milner's low ball to cut the lead further after the goalkeeper had pushed the forward's first shot back to him.
The losers take some pride from the day. In stoppage time they were pressing for an equaliser and at the full-time whistle the delight of the visiting United supporters was topped up with relief that their team had stopped short of bungling what looked a certain victory.
There is no cause to upbraid City. Having floundered in the Champions League, they had the Premier League as the sole trophy of compulsive interest to them. No one supposes they are on the brink of capitulation but they look a different side. City bore no relation to the line-up who, until recently, could not help but score goals in abundance.
Fluctuations are to be anticipated but this experience is new to the contemporary City. They have to reckon with factors such as the dip in form of David Silva. It seemed certain he would be elected the 2012 footballer of the year. He is too good not to be prominent again before long.
Mancini, for the moment, is inconvenienced if not exactly embattled. While his side struck twice there is no longer a sense that there will be a spate of goals to sweep away the opposition. That confident mood never lasts long enough for any club's liking. This may have been an FA Cup tie but United will have left the Etihad Stadium feeling much more optimistic about the league than they had at St James' Park four days earlier.