Talking about his future at manchester united, in my opinion he have a 50/50 chance to remain the main goalkeeper in the red devils squad next season, if Amorim recruits a new goalkeeper in the transfers market later.
My guess is that they don't have an alternative goalkeeper. They are rare and the only thing Amorim can do about it in the middle of the season is to express his trust for Onana or otherwise he would further weaken him. He has made mistakes repeatedly, but I think he was more stable recently apart from the game against Plzen, but they still won the game though.
It comes down to whether Manchester United is able to sign a new top goalkeeper or not. I doubt it because they are very likely going to not play in the Europa League next season and I don't think any great goalkeeper would decide to go to Manchester United if they can't play any of the bigger tournaments.
Onana loses his concentration sometimes and then he looks as if the lost orientation especially in the box. It's so vital for a goalkeeper to avoid the easy mistakes. The problem is if he defends five balls in a spectacular way but then makes a stupid mistake, the game can still be lost.