The keeper was at fault for both goals as United were held to a 2-2 draw by Lyon in the first leg of their quarter-final tie on Thursday (10.04.25) but the manager is standing by the man between the posts.
Amorim told TNT Sports: "There's nothing I can say to Andre in this moment that will help him.
"The most important thing is to be natural and then when the time comes, I will choose the best XI to play. But I'm really confident in Andre.
"We continue to do the same thing. Training, seeing the games, trying to choose the best 11 to win every match."
Amorim later suggested that he had "made more mistakes" than Onana - who was branded "one of the worst goalkeepers in Manchester United history" but former Red Devils midfielder Nemanja Matic before the game - during his early months in charge at United.
The Portuguese coach said: "It can happen.
"If you play football, a lot of games, you can make mistakes. If you look at the season, I've made more mistakes than them in these last games and during this last month.
"The best thing is to look at the goals [that Onana conceded], the actions that Andre had in the game, and to talk to the [goalkeeping coaches]. That's the best thing to do, to help every player to show what happened in the game and what we need to improve, and then put it to the player again."