Manchester City manager Manuel Pellegrini was left nonplussed after his ailing side were hammered 4-2 by bitter rivals Manchester United at Old Trafford.
City are now in a desperate fight to qualify for next season's Champions League, as a resurgent United blitzed them to make up for their 1-0 defeat at the Etihad earlier in the season.
Pellegrini, whose side had won four derbies in a row before this game, is under massive pressure to justify continuing in the job he has held since the summer of 2013.
He said: "I don't understand why a team only plays so well in the first 20 minutes. We dominated, created chances, we had the ball. At 1-1 United started growing and their third goal gave them the confidence they did not have before.
"It is not just one problem, the same team played well for 20 minutes. Maybe it's a lack of class at this moment. I don't know."
The Chilean's confusion is unlikely to go down well with Khaldoon al-Mubarak, the City chairman, with whom he will meet at the end of the season to discuss a campaign he himself has described as "garbage".
And with City now hamstrung by Financial Fair Play and with the oldest squad in the Premier League, they look in a poor position to be able to rebound next season.
"It is very difficult to be in fourth place. We have difficult teams in the Premier League. We have just four points behind United, five points with Arsenal," Pellegrini continued.
"We must fight until the end of the season. For the club, for me, for the players it is very important to be in a position for the Champions League. We have six games more to try to do it. We had been in second place all season. We have six games to go."
Louis van Gaal, whose side have now moved four points ahead of City and retain faint hopes of the title, declared himself "very happy for the fans".
He said: "It's three points! We are four points ahead of fourth in the table and I am very happy for the fans. 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."Reuse content