Amorim says Casemiro did everything he wanted on the day in midfield.
"The important thing with Casemiro is that he won everything with Real Madrid," said Amorim.
"He wasn’t playing because I understood that in that moment he wasn’t the best option for how we want to play and he continued to work.
"I look at my players in the same way. If you improve what we ask of them, they have the opportunity. If they play well, they continue to play.
"That was the situation for Casemiro. The credit is all the player. He is our player and will continue to be our player.”
On the team's performance, Amorim added: "The important thing is if you compare this game to that against Arsenal, we had some moments when we did the same a little bit to City. And against Arsenal we didn’t do: clearly it was transitions.
"Today it was a little bit different. We are improving but to play in this way, that they played for many years, is going to take a long time.”