United were beaten by goals from Brennan Johnson, Dejan Kulusevski and Dominic Solanke.
Ten Hag later said, "To concede a goal after two or three minutes like that, it did something to our belief. Totally unnecessary. To concede a goal like this, the centre-half stepping in and passing the whole team over the right side and then the winger tapping in at the far post. Same mistake as in the midweek against Twente, it should be stopped in the first moment.
"From that moment on we were stressed on the ball. We could not find the triangles or the switches, we had some good moments, but from that moment we could not get a foothold. And we can do better than that. Even after such a mistake from us we should stay calm.
"We had chances and our moments to get back in the game, with Joshua Zirkzee and Alejandro Garnacho when we still had 11 players and then after the red card too we had moments.
"We had our chances before half-time and in the second half with 10 men we had chances as well. It definitely had an impact on the game. I don't think it was a red card."
"First we have to deal with this and show resilience. This is not good enough. We have to accept this and get better. Focus on the next game and do things better than we did today."