Jose Mourinho criticised his Manchester United players for being "really below the normal level" as they slipped to a 2-1 derby defeat to Manchester City at Old Trafford.
Kevin De Bruyne and Kelechi Iheanacho scored during the first half as City threated to run riot, but an error from debutant Claudio Bravo in the visitors' goal allowed Zlatan Ibrahimovic to reduce the deficit.
United pressed for an equaliser that never arrived during the second half, with Bravo's erratic showing giving them continued cause for encouragement, but Mourinho was left to reflect on three Premier League points lost before the interval.
"I think in the first half, they were much better than us," he told Sky Sports.
"We started the game badly. We started the game with some players really below the normal level, in terms of their concentration and their mental ability to play a game of this speed.
"You need to be sharp, the way you think and the way you play, we had a few players below the level. The team paid for it.
"In the first half, they were in control. The second half was completely different. We were in control, much more dangerous and had our chances to equalise."
Mourinho handed a full debut to Henrikh Mkhitaryan and recalled Jesse Lingard to his starting line-up, but both made way at the break and the former Chelsea boss stated he made selection errors.
"I had two or three players in the first half where, if I know what is going to happen, obviously I don't play them," he said.
"But this is football, and sometimes players disappoint managers, and sometimes players give us great surprises.
"I thought, because of the characteristics of the game, I thought we could hurt them playing with some individuals with some qualities.
"They didn't give me what I wanted, therefore it's my fault. I'm the manager and it's always my fault, because it's my choice."
Somewhat customarily, Mourinho chose to question the role of the referee in one of his team's defeats, claiming Bravo should have been sent off for a lunging tackle on Wayne Rooney, whose later cross struck Nicolas Otamendi on the arm when the City defender had his back turned.
"Obviously, we are very disappointed with the result, but I am also very disappointed with two decisions of Mark [Clattenburg]," he added.
"It's a penalty and a red card for Bravo. If you tell me that that's not a penalty and a red card, I tell you that, for sure, outside the box it's a free-kick and a red card. Inside the box has to be the same.
"Even the best referees can make mistakes and he made a big one. The second one I also think is a penalty.
"Otamendi, it's a penalty. You can tell me he has no intention, I think he has intention. In spite of his arm being behind his body, he knows that the cross is not going in his face, it's going in his back, so he drops his elbow. It's two penalties against us."