Manuel Pellegrini accused his Manchester City team of only playing for 20 minutes and said he was concerned by the spirit of his struggling side as they were condemned to defeat in the Manchester derby.
City were beaten for the sixth time in eight games as, despite taking the lead through Sergio Aguero, they lost 4-2 to Manchester United at Old Trafford.
They are now four points behind their neighbours and 12 adrift of Premier League leaders Chelsea, who have a game in hand.
And Pellegrini, who admitted United deserved their victory, was worried by the way City failed to build on their fine start.
"We must play not for 20 minutes but the whole game in the same way," he said in a news conference. "I don't understand why a team only plays so well in the first 20 minutes. We dominated, created chances and we had the ball."
The Chilean said he was accountable for City's poor run, adding: "I always think it is my responsibility."
City have lost eight of their 17 games and, after being level on points with Chelsea on New Year's Day, could be 15 points behind them if Jose Mourinho's team win their game in hand.
"We are not in a good moment," Pellegrini added. "We have a bad 2015 and a very bad week."
Pellegrini was worried by the way his side's heads dropped after Juan Mata put United 3-1 ahead, while arguing the Spaniard was offside and saying it is harder to play when two goals behind.
"Maybe it is something we can be concerned [about]," he said. "There are lot of reasons why maybe the spirit of the team is not the real spirit."
Asked if he needed to question his own performance, the Chilean said: "I always think it is my responsibility, always."
Pellegrini shrugged off speculation he will be sacked, adding: "I am not talking about my position. It is not important."
Information from Press Association was used in this report.