I quite agree with the 4 players who will be sold because apart from their age, they are now starting to be no longer relevant to the attractive game that Amorim wants to bring. Rashford performance is also not too prominent but Onana can still be negotiated because he only needs motivation and is given high trust by Amorim. With the results of the player sales that if successful, Man United has the opportunity to improve several vulnerable positions.
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.