Onana isn't as bad as you put it. His performance was bad last season because of poor defense but this season, Onana is the goalkeeper that has made the highest save in EPL currently. The mistake that he made in their last match was a mistake that no one is above. Amorim believe in him, and he is fit to be United number one goalkeeper. Onana has saved United ass so many times this season.
If Onana is still willing to learn from his mistakes in the Europa League match a few days ago, of course he still deserves to be trusted as the main goalkeeper in the Manchester United team this season, but if something like that happens again in the upcoming matches this season. I think Amorim should look for another goalkeeper to be the main goalkeeper at Manchester United because his last mistake directly resulted in a goal for the opposing team and that is clearly unforgivable by anyone.