Onana performance varied significantly from a match to another to be honest. He is still young to be ejected from the team but his mistakes cost the team many goals. Nevertheless, the defence line in front of him is defective and sometimes useless so he has to take the blame instead of them in eyes fans. I don't know what's the plan of Manchester United because he isn't playing all games for the moment, from the last one to the friendly ones and there is talks about Manchester United chasing James Trafford from Burnley which are asking for more than $20m to let him go.
This seems like a "small" problem, but in reality for a position like goalkeeper it is a huge problem. For example, a striker can have several unsuccessful games in a row and it will not be very noticeable, but when a goalkeeper has one unsuccessful game out of several, everyone notices. Plus it affects the confidence of the defensive line, and therefore the whole team. What everyone wants from goalies is not individual bright games, but on the contrary, an even level throughout all games where there is a guarantee that on average they maintain their level. This cannot be said about Onana, although he has many good games.