Manchester United boss Louis van Gaal was delighted after the 4-2 win against Manchester City but was baffled by his side's approach to the game.
It is United's sixth consecutive win in the Premier League as they continue their quest for a second place finish.
They are only one point from Arsenal but with tough away games coming up against Chelsea and Everton at the end of the month.
"It's three points! We are four points ahead of fourth in the table and I am very happy for the fans," van Gaal said.
"I have seen what it means for them. They always support us when the results were not good.
"It's the best atmosphere I have seen here. A fantastic result.
"I must bring Michael Carrick off the pitch with an injury which was a bit stupid from me as some players didn't know we only had 10 men. That was not so smart!
"We started very badly. It was not Manchester United, we were very nervous. I don't know why. We had no pressure on the ball. I don't know why. But when we scored the goal, from that moment we had more confidence.
"The second half was much better."