City won 3-0 through a double from Erling Haaland and Tijjani Reijnders' effort.
Nuno remarked afterwards: "I think we started bad, we started really bad.
"We conceded really early, we made things hard in terms of the game plan, the way we wanted to do it. The first spells of the game were not good, we lost balls, we didn't go out.
"We were much improved in the second half, much improved. Eventually, with one of those chances, we could change the game around and get us back in the game.
"The performance was not so good in the first half, but better in the second half, so the biggest challenge is to try and do it from the beginning."
He also stated, "We had belief all the game.
"It's very hard to play against City, they are a very good team. The quality of their players is difficult in our defensive tasks.
"But the belief was there, and especially with the support of our fans. They were incredible, even when we were losing, and they keep believing and try to support the boys.
"We have to keep this idea that everything is possible and we require improvement."