Having bounced back from alarming defeats to Brighton and Brentford with four successive Premier League wins, the Red Devils were brought down to earth with a bump at the Etihad Stadium.
Erling Haaland and Phil Foden both scored hat-trick as City ran amok in the highest scoring Manchester derby.
Substitute Anthony Martial’s brace complemented a superb second-half strike from summer signing Antony for United, but a number of fed up visiting fans had already headed for the exits by that point.
“I also told the players (it was a) lack of belief,” manager Ten Hag said. “I’ve seen some highlights. It was obvious that we are defending not (on the) front foot, we let them play.
“We were, in possession, not brave enough, made technical skills mistakes, decision mistakes and then you get hammered.”
Put to the Dutchman that professional footballers should already have belief, he said: “I think often it’s the other way around.
“So, for instance, in the last five games we bring that attitude on the pitch. We have a strong belief and convincement, we bring opponents problems.
“Today it was the other way around but I think it’s normal in our process that the pattern has to go up and I think I see clearly it goes up.
“But you will have setbacks during that process and we have to learn from it but point it clearly out.
“We can walk away from it but if we do it we will not get the solution.
“We will be very clear and then we have to move on. We will carry on to the next game.”
Ten Hag says United players have to accept that criticism and produce team performances akin to those shown in the victories against Liverpool and Arsenal.
The Dutchman acknowledged that the fact they had not played a league game since winning the latter 28 days ago had an effect but said that was no excuse “when you don’t bring the right performance”.
Instead, Ten Hag underlined that the lack of belief undid their rules and principles at the Etihad Stadium, where the meek loss will not lead to the kind of punishment he dished out after the humiliating loss at Brentford.
“The punishment had a reason – they outrun us so we didn’t work hard enough,” said Ten Hag, who made his players run 13.8km the day after that defeat – the distance the Bees had collectively outran them by.
“Today is also mental but it’s a different mental issue, it’s like belief. It’s not always about punishment.
“Of course, we have to criticise, we have to criticise each other, so I didn’t get the message right to the players quite clearly to get the belief on the pitch and from then on we didn’t play our game, our way of playing.
“That’s disappointing but it’s good it happens against City. But then you get quite clear what the problems are when you don’t bring it.”
United return to Premier League action at Everton next Sunday, having travelled to Cyprus for Thursday’s Europa League clash against Omonia Nicosia.
Cristiano Ronaldo will be hoping to have a role in those matches having been left on the bench during the damaging defeat at City.
“I wouldn’t bring him in because we are 4-0 down out of respect for Cristiano, for his big career,” Ten Hag added.
“And the other thing, there was then the advantage I could bring Antony Martial (on). He needs the minutes but I don’t want to point it out like that.