Ten Hag also says they let their fans down after the humiliation.
He said, "It's quite simple, it's a lack of belief. When you don't believe on the pitch then you can't win games, that is unacceptable. We get undisciplined at following rules and you get hammered, that's happened today.
"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.
"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.
"From the first minute I felt it and 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.
"In this moment I can't think about positives. We let our fans down, we let ourselves down and we're hugely disappointed."