Substituted before halftime due to Patrick Dorgu's red card, Garnacho didn't take his place on the bench and instead went straight to the home dressing room.
The issue has since been cleaned up, though Amorim admits "perceptions" at United are important.
He explained, "It was funny because in the next day he came to me, to my office.
"I did some investigations, and he went to the dressing room, he changed his clothes because he was wet.
"He watched the game in the different, not on the bench. In the end of the game he was there, then he went home, so there's not an issue there.
"But I told them that in Manchester United everything is important, and the perception in a big club is really important, so he's going to pay a dinner to all the team, and that's it.
"I think he understands. He was subbed, he was playing well and has to go off in that moment of the game. It's hard to deal with everything but they have to deal with everything. I just try to help them be better footballers."