Erik ten Hag said Manchester United got what they deserved in a 6-3 thrashing at Manchester City because they didn't believe they could beat the Premier League champions.
For the second time in just seven league games under Ten Hag, United trailed 4-0 at half-time as City ran riot.
Erling Haaland and Phil Foden completed their hat-tricks after the break and only Anthony Martial's late double saw United avoid a record-equalling defeat in the Manchester derby.
"It's quite simple, it's a lack of belief," Ten Hag said. "When you don't believe on the pitch then you can't win games, that is unacceptable.
"We got undisciplined at following rules and you get hammered, that is what happened today."
United had bounced back from an embarrassing 4-0 defeat at Brentford in August to win their previous four league games.
But any hopes of showing they had closed the gap on City were eviscerated during a first half in which Pep Guardiola's men could have scored even more.
"For me it was a surprise. We were not on the front foot, we were not brave on the ball and there were spaces to play but we were not brave enough," added Ten Hag.
"All the credit to City, but it's nothing to do with City. Our performance was not good. It had to do with the belief as individuals and as team."
United did show some fight in the second period as Antony fired home his second goal since joining Ten Hag in moving from Ajax to Old Trafford before Martial struck twice in the final six minutes.
But the decision to leave Cristiano Ronaldo on the bench for the full 90 minutes raises further questions on what role Ten Hag foresees for the five-time Ballon d'Or winner for the rest of the season.
Ten Hag said he overlooked Ronaldo "out of respect" as he did not want to throw the 37-year-old on with United chasing shadows.
But the Dutchman was pleased with the reaction his side showed after the break.
"In this moment I can't think about positives. We let our fans down, we let ourselves down," he said.
"At half-time we changed things and we changed attitude. We saw a different Manchester United after half-time and we scored goals, we created some more.
"On the ball we were more brave, we came in the opponent's half and we scored three goals."