Casemiro scored and was sent off before halftime of the 2-1 win at Old Trafford.
Afterwards, Amorim said: "With Casemiro, that’s the pressure. The guy has five Champions Leagues. I feel more pressure in some young guys sometimes.
"But maybe it’s because Casemiro cares - we scored one and then he has that tackle. Then they have maybe one time that they passed the half of the pitch and we want to win the ball right away.
"So I think sometimes it’s a little bit, not the pressure, they want it too much in that moment. That is not a bad thing.
"I don’t remember a big play that we did. It’s not about that in this moment. It’s about the second ball, the first ball, the aggression, the speed. Even Case with the sending off, but it was really aggressive.
"Of course, he needs to do better in that moment, in that place. But we show that today, since the beginning of the game, we show that we wanted to win really, really badly.”