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.
I also admit that Onana is actually able to give the best contribution to United. And last season Onana had to fight alone when the defenders could not contribute to Onana who seemed to hold back every attack from the opponent.
However, even so, I will not judge Onana too much because in fact MU best guard is only De Gea but unfortunately we have to forget about that and can only hope that Onana can develop his talent to be able to have talent like De Gea.
I believe that if Amorim later makes a reshuffle, Onana will definitely be maintained and Amorim will actually sell some defenders and replace them with new players.