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."
- You can now get Man Utd news notifications in the BBC Sport app - find out more
Former Manchester United defender Gary Neville told Sky Sports the defeat was a "reminder of how far behind" they are compared to City, who sit one point below leaders Arsenal in the Premier League table.
Since United last won the league title, City have won it five times, including the last two in a row.
"It is a process. It is one game. Maybe against other teams the problems would not come but against Manchester City they will," added Ten Hag.
"I want to change the head and the attitude from the team. There was a reaction in the second half. We were more brave and scored three goals. Only from mistakes you learn."
City imposed their dominance very quickly and led 4-0 by half-time. Former England defender Jonathan Woodgate said on BBC Radio 5 Live it was "absolutely embarrassing for Manchester United".
Ex-United captain Roy Keane added on Sky Sports: "I am shocked at how poor United have been. They just didn't start well. It is brilliant from Manchester City but it is like the game is too big for Manchester United. The occasion has got too big for them.
"I can't believe how bad Manchester United have been. They should be embarrassed."
- Our coverage of Manchester United is bigger and better than ever before - here's everything you need to know to make sure you never miss a momentEverything United - go straight to all the best content
Source: bbc.com