Manchester City boss Manuel Pellegrini has confessed that he is confused by his side's slump in performance in their 4-2 defeat to Manchester United at Old Trafford.

City started the brighter of the two sides as they threatened the United goal on several occasions in the opening minutes before Sergio Aguero converted David Silva's cross from close range.

However, the Red Devils quickly rallied to equalise through Ashley Young and the home side soon edged ahead as Marouane Fellaini headed home.

From that point it was all one-way traffic as goals from Juan Mata and Chris Smalling secured the win for Louis van Gaal's side, despite a late strike from Aguero.

Pellegrini told Sky Sports News: "I think they were the best team. We started very well and in the first 20 minutes we could have controlled the game. After the second goal we lost the way we were playing and United scored more goals.

"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."

City are now four points behind United in fourth place in the Premier League after their defeat at Old Trafford.