Jose Mourinho felt Sergio Aguero's reaction to a clash of heads with Marouane Fellaini played a role in the Manchester United midfielder's red card.
Just 19 seconds after being booked for a tackle on the Manchester City striker, Fellaini was penalised for another foul on Aguero, who he headbutted in the aftermath.
Referee Martin Atkinson consequently showed the Belgium international a straight red card in the 84th minute, with United having to hold on for a 0-0 draw.
While Mourinho said Fellaini was partly to blame for his sending off, he suggested Aguero's reaction helped convince Atkinson to dismiss the midfielder.
"I didn't watch it on TV. I spoke with Marouane and he is very disappointed, he feels it is a red card because he is Marouane Fellaini," the United manager told Sky Sports.
"I spoke with Martin who said in his opinion it was a red card. I didn't watch but I can guess it is a bit of a red card and a bit of a very smart, very experienced Argentinian player."
Mourinho was pleased with United's first-half display despite the absence of players including Zlatan Ibrahimovic, Paul Pogba, Juan Mata and Marcos Rojo, but acknowledged they had to play for a point as their attacking threat dwindled.
He said: "We wanted to do more in terms of attacking and to explore more of the counter-attack. In the first half we had good control but the second became more difficult.
"It was even worse with one player less so we had to make a decision to fight hard to take a point.
"They started strong [in the second half], pushed hard and at some times we could not move the ball well. I promised not to speak about the players who weren't playing so I won't.... but we just missed a little bit of that quality in midfield to connect with the attacking players."
United are now unbeaten in 24 successive Premier League matches – equalling their longest run in a single top-flight season – and Mourinho praised his team's resilience.
"We showed exactly that against a good strong team that wants to win at home, playing with 10 men," he said.
"I didn't have any midfielders on the bench. The team is in trouble, the team has a lot of problems and to play a derby like we did shows resilience and character from the players."