Louis van Gaal angrily branded Chris Smalling “stupid” after his dismissal helped City beat United in the Manchester derby.
United were holding their own against their local rivals, but eventually went down 1-0 thanks to a second-half strike from Sergio Aguero.
City’s task was made much easier by Smalling, who received two yellow cards in eight minutes just before half-time.
Referee Michael Oliver showed Smalling a yellow card for a cynical block on Joe Hart as he attempted to kick the ball down field from his hands.
And the England defender slid in on James Milner soon after, leaving Oliver with no option but to issue him with a second yellow.
Van Gaal said: “In a derby you have to be careful - the second yellow card is a stupid yellow card. As a player you have to control your aggression. I didn’t see the first yellow but with the second, you know you already have a yellow, so have to handle it differently. I said that to the players.
“You have to deal with that. You cannot do what he has done with the second yellow card. That is not very smart.”
When asked whether Smalling had apologised to his team-mates, Van Gaal said: “He said it very humbly. You can only accept it. But nevertheless it was in my opinion not so smart.”
Van Gaal’s anger is likely to have been heightened due to the fact that United are already severely depleted in defence.
Marcos Rojo will join Phil Jones and Jonny Evans in the treatment room this week after he hurt his shoulder sliding in for a tackle in the second half.
“He has dislocated his shoulder,” said Van Gaal, who ended the match with Michael Carrick and Paddy McNair at centre-back. “He won’t be available next week. We have lot of injuries in defence.”
City launched wave after wave of attack at the United goal following Smalling’s sending off and the visitors finally crumbled when Aguero turned home Gael Clichy’s cross in the 63rd minute.
City should have added more to their tally, but they suffered a bout of nerves towards the end of the match, when Marouane Fellaini and Van Persie had good chances to level.
That did not matter much to Manuel Pellegrini, who was happy to see his team end their three-match winless run.
“I am very pleased with the performance. It is always important to win a derby, especially against this Manchester United team which is a great team with great players,” Pellegrini said.
“We played really well all the match except the last 10 minutes with less intensity but I am really happy about the result.”
Meanwhile, Aston Villa’s five-game and 15-minute wait for a goal ended when Andreas Weimann opened the scoring against Tottenham at Villa Park.
However, Spurs came back and after Christian Benteke was shown a straight red card by referee Neil Swarbrick after a tussle with Erik Lamela in which the Belgium international raised his hand to his opponent’s face Nacer Chadli fired home Lamela’s corner, before substitute Harry Kane’s deflected last-gasp free-kick claimedthe win.