Ronaldo was an unused substitute at the Etihad Stadium despite United finding themselves 4-0 down at half-time.
Victor Lindelof, Anthony Martial, Fred, Casemiro and Luke Shaw all came off the bench ahead of Ronaldo, with Ten Hag insisting afterward the 37-year-old did not deserve to be subjected to humiliation at the hands of Pep Guardiola's champions.
"I wouldn't bring him in out of respect for Cristiano, for his big career," Ten Hag said.
"The other thing was the advantage that I could bring on Anthony Martial. He needs the minutes, but I don't want to point it out like that."
Martial came off the bench to score twice after Antony had pulled one back in the second half, but Ten Hag said the damage had been done during a disastrous first half and accused his players of showing the wrong attitude from kick-off.
"It's quite simple, it's a lack of belief," Ten Hag added.
"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.
"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 a team.
"From the first minute I felt it, and at half-time we changed things and we changed attitude.
"In this moment I can't think about positives. We let our fans down, we let ourselves down and we're hugely disappointed."
Meanwhile, City have extended their unbeaten start to the season and have now scored 29 goals in just eight league games.
Guardiola insisted there was still room for improvement but praised Haaland after the Norwegian improved his record this season to 17 goals in just 11 games.
"The quality we have alongside him helps him to score goals," Guardiola said. "But what he has done, I didn't teach him once. When the ball is right, and he attacks the box, how he moves behind the central defender away from the action, he has incredible instincts that the ball will arrive there and it comes from his mum and dad, he was born with that and just to help him produce more goals.
"What I like in the last period of the game is that he is involved. He said, 'I prefer to touch the ball five times and score five goals.' I don't like that, I want him to touch the ball more and more, so he became a football player to score goals.
"I like that he's part more for the situations, but don't forget his biggest talent is to put the ball in the net."