Both Erling Haaland and Phil Foden scored hat-tricks in a 6-3 victory, the scoreline of which flattered United thanks to two late goals from substitute Anthony Martial.
Ronaldo was left on the bench as Ten Hag made five changes, the first of which came in the first half when United were already 4-0 down.
Asked why Ronaldo was not among the introductions during the game, Ten Hag suggested he was left on the bench to avoid the ignominy of being involved in a heavy defeat.
When asked why Ronaldo was not brought on, Ten Hag replied: "I wouldn't bring him in because out of respect for Cristiano, for his big career.
“And the other thing was the advantage I could bring Martial, he needs the minutes but I don't want to point it out like that."
Ronaldo has come on in each of United's previous four games, all of which were victories, and Ten Hag hinted ahead of the game that the Portuguese could feature.
As it was, he was left on the bench as Marcus Rashford lead the line, with Martial coming in the second half when the game was already lost.
Asked to explain the heavy loss, Ten Hag told BBC Sport: "It's quite simple: it's a lack of belief. 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 team."
He concluded: "In this moment I can't think about positives. We let our fans down, we let ourselves down and we're hugely disappointed."