Marcus Rashford's eighth-minute strike gave United the lead but two goals from Phil Foden and a late effort from Erling Haaland sealed the win for Pep Guardiola's side, who are a point behind leaders Liverpool.
United are set to finish below their rivals for an 11th consecutive season but when asked if the latest defeat was a reminder of the gap between the teams Ten Hag told reporters: "No I don't think so, absolutely not.
"We have many problems now in injuries and still we had an opportunity. By a very small margin. We could have scored the second goal. You see it (the gap) is not that big.
"When we have everyone on board we can be really competitive and also we showed, for instance, in the FA Cup final against them when it can be really close. But don't forget this, City is, at the moment, the best team in the world."
United, who are sixth, next host Everton in the Premier League on Saturday.
