Manchester United manager Erik ten Hag said there was a "lack of belief" from his side in their 6-3 defeat to rivals Manchester City.
Despite winning their four previous Premier League matches, United were heavily beaten by City and trailed 4-0 at half-time.
Erling Haaland and Phil Foden scored hat-tricks for City while Anthony Martial and Antony netted for United.
"When you don't believe on the pitch, you can't win games," said Ten Hag.
"It's quite simple, it's a lack of belief. That is unacceptable. We get undisciplined at following rules and you get hammered, that's happened.
"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.
"In this moment I can't think about positives. We let our fans down, we let ourselves down and we're hugely disappointed."