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.