Onana can't become a stable goalkeeper, which is very important for any team that wants to achieve great results, I think that Manchester United should have a goalkeeper of a much higher level, because when in some games the goals conceded are the goalkeeper's fault, then this problem needs to be solved, and it should have been solved a long time ago, long before Amorim arrived. This is not the only problem of the team, and often there are even more of them than there were, I think to make Manchester United great again, a lot of changes need to be made.
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.
While I do agree that United has terrible defense and that was one of the reasons why they conceded that many goals, we have also seen Onana make a lot of mistakes as well, easy saves turning into silly goals was a big issue last season and feels like he does that less this year and if they can wait for him to recover a bit more, he can become a decent goalkeeper but it's certainly clear that they got him for too much if they have to wait 2+ seasons for him to be any good.
Right now Onana is not an elite level goalkeeper, even with elite level defense, he would have been terrible. But that doesn't mean they are doing terrible, I am sure that he is going to get better. With better defense on the way as well, I am sure they will do fine.