The former Real Madrid star has been linked with a move away from Old Trafford for the past year.
Since Amorim's arrival, Casemiro has made only six appearances and hasn't played in United's last four league games.
On Casemiro, he said: “Sometimes it’s a lot of that. It’s also a player’s moment. We understand that Casemiro has other things nowadays.
"The intelligence he has, understanding the game, understanding where the ball is going to fall but we’re in a league that I can see, even in European competitions, the difference in intensity is big.
“And so, I feel that this team also needs players with a very high intensity. And sometimes we don’t have that and sometimes that difference in characteristic can lead one player or another to play.”
Amorim added: “Even the fact that the group has a certain characteristic as a group. I needed some players in certain positions with a slightly different pace. But we all know the quality Casemiro has and everything he won. And so, I have nothing to say about that. It’s just a choice.”