Manchester United coach Erik ten Hag said a lack of belief was the reason his side capitulated so badly in their 6-3 thrashing at the hands of local rivals Manchester City on Sunday.
United came into Sunday's derby clash at the Etihad Stadium on the back of four successive league wins, a run that included victories over Arsenal and Liverpool.
But hat-tricks from Erling Haaland and Phil Foden earned City a rampant victory, with the winning margin not giving a true reflection of the gulf between the old foes.
"We lacked belief from the start," Ten Hag said. "In the last five games we had a strong belief, today it was the other way around.
"We are not defending front foot and we let them play. In possession we were not brave enough, made mistakes and bad decisions -- then you get hammered.
"There are never excuses in top football, you have to bring results, but it is normal in our process we will have setbacks. We just have to learn from it."
Ten Hag also explained why substitute Anthony Martial was the man he turned to in the second half to try and get United back in the match, rather than Cristiano Ronaldo, and why big-money signing Casemiro was again overlooked for a starting spot.
"I wouldn't bring him (Ronaldo) in out of respect for his big career," Ten Hag added. "Anthony Martial needed the minutes and he scored two goals.
"When we signed him (Casemiro) we started to win. It is about the team, and the team was doing really well.
"Scott McTominay has been performing well, but I am sure Casemiro will be important for us in the long run, and find himself in the team in the short term. That will come in a natural way."