The holders are out. Despite a wonderful physical effort by a weary Manchester City, they could not make up the lost ground caused by the events of the first half. Wayne Rooney's first goal was superb but Chris Foy's decision to send off Vincent Kompany, in keeping with his erratic decision-making during the Stoke City v Tottenham Hotspur game last month and QPR's win over Chelsea in October, was the main reason two heavyweights were unable to serve up a lunchtime feast of football.

With Sergio Agüero a lone forward for City and Rooney dropping to aid his midfield, United comfortably dominated the first period with their one-man advantage. Danny Welbeck's pivot and technique and Rooney's penalty appeared to give United an unassailable lead. However, Roberto Mancini's tactical revision, which was reminiscent of André Villas-Boas's rejig against QPR for Chelsea, kept supporters on tenterhooks. Moving Samir Nasri closer to Agüero, introducing Pablo Zabaleta's athleticism and Stefan Savic alongside Joleon Lescott, he sacrificed his two cleverest creators, David Silva and Adam Johnson. I wonder if City might have snatched a dramatic draw if Silva had stayed on instead of the casual Nasri.

Regardless, City's back line now contained two attacking full-backs, with Aleksandar Kolarov forcing Antonio Valencia to defend, but were vulnerable when United attacked down the right. United should have had a penalty when Kolarov fouled Valencia – another error by Foy. City's midfield, which was supported at every opportunity by Kolarov, transferred an attacking mentality to the home side and gave them some self-belief. James Milner's indefatigable effort from central midfield was noticeable.

City had to move the ball steadily. The longer passes allowed United to win the ball back on a day when their defence was nervous despite their lead. The changes initially suggested Mancini had come to terms with a fait accompli, accepting a 3-0 defeat and settling for damage limitation. United were composed when they had possession in midfield, as Rooney intelligently dropped deeper to help them keep the ball. When City did regain possession, they had to build moves with care.

When Agüero's sharp finish brought the score back to 3-2 it was still a mammoth task for City, provided United used their experience. With Rooney further forward and Danny Welbeck replaced by Anderson to give the midfield fresher legs, City grew even wearier but continued to battle bravely.

I have long thought that when teams are reduced to 10 men, they must keep forwards upfield or they give unopposed possession to opposition defenders and it is impossible to stop a stream of attacks building up. Here Mancini kept two men forward and gambled his defence would stay firm. In truth Costel Pantilimon had little to do in the second half and United were not looking to increase their lead, rather to make the most of the fortune they enjoyed in the first half.

Many years ago, Wimbledon, who were the kings of direct football, were often reduced to 10 men, but more than one of their managers kept the opposition at bay by using two or three players in attack and, in effect, playing a 3-4-3 formation. One could not expect City to suddenly play direct football and bypass midfield like Wimbledon; that style is foreign to them, but also they did not have the attacking personnel required to implement such a system.

This was a fascinating game because of City's rally in the second half, and both teams will take positives from it. However from a neutral point of view, Foy's decision to send off Kompany changed the dynamic of the contest and frustration for City is understandable when defeat comes about in such circumstances.