The Argentine, an outcast since September after falling out with the club, came off the bench with City trailing 1-0 to Gary Cahill's deflected shot and his moment of class in the 85th minute allowed City to move to within a point of United.
Sergio Aguero had levelled from the penalty spot for City who have won all 15 home league matches this season.
Tottenham Hotspur drew 1-1 at home with Stoke City in their first game since their FA Cup tie against Bolton Wanderers was abandoned following Fabrice Muamba's cardiac arrest.
Rafael van der Vaart scored a late equaliser for Spurs to end a run of three league defeats but they fell below north London rivals Arsenal who won 1-0 at Everton to climb to third.
Arsenal have 55 points to Tottenham's 54 with Chelsea, who host Tottenham on Saturday, on 49.
Liverpool suffered a late collapse in west London, throwing away a 2-0 lead to concede three late goals and lose 3-2 at relegation-threatened Queens Rark Rangers.
