Onana is not underperforming; rather, he's performing to his best performance; I don't think Manchester United is letting him go that easy because he is not part of Manchester United's problem. He won't defend the goalpost and act like a defender at the same time. He will only act like the goalkeeper, that's it. Nothing above that.
The problem of Manchester United is from their players. If the right players are brought into the squad, Manchester United will have more match wins than loss matches.
Trust me mate, he is kind of underperforming at Manchester United.

Do you remember his times at Inter? He was such a solid goalkeeper back then.
He was even like the goalkeeper with the most clean sheets in the Champions League one season. That was impressive! This must also be a factor drawing the Red Devils' attraction.
However we can say one thing about Onana. He isn't consistent throughout his career. He had bad times earlier than Inter too for example.
Most players, even the best of the best, can be very impressive with a team then fail hard in another one and I can give you a lot of examples starting from Cristiano Ronaldo once he left Real Madrid and joined Manchester United and Juventus, he failed there. Mbappe isn't that excellent in Real Madrid comparing to what he was with PSG. Neymar is another example. In goalkeeping, Donnaruma was excellent in Milan then dropped in PSG and right now he is getting good time.
Onana isn't any different from these and if we added the failure of the whole team and the defence line then the image will be clear..