United went into the game on the back of a four-game winning run in the league, including scalps over Liverpool and Arsenal, but were undone by hattricks from Erling Haaland and Phil Foden.
Pep Guardiola's side went into the break with a 4-0 lead, the second time Manchester United have gone in at half-time four goals down this season, with the first coming in their second game of the season away at Brentford.
By the time substitute Anthony Martial grabbed a brace in the second half, adding to Antony's sweet strike, the game had been already settled and only brought a measure of respectability to the scoreline.
Asked what went wrong for United, Ten Hag told Sky Sports: "A lack of belief.
"I'm surprised, why we didn't bring it on the pitch. I've seen – since, obviously, four weeks ago – a different team, a different spirit, a different fight and a level of commitment.
"Today we didn't have commitment. From the first minute on, we were not on the front foot, we were not brave in possession"
Pushed on why he thought United fell short of their recent standards, the Dutchman added: "It's always difficult to find out.
"I have to talk with my players and find out. They know they got better [in recent games], I saw the performances against Arsenal and Liverpool.
"But if you don't fight, which they didn't, then you have a problem against a team like Man City."
The Red Devils are back in league action away at Everton this weekend.
