He's currently the third choice goalkeeper for Manchester United, as much as he's still with Manchester United, he will forever be behind these players. He needs to leave on loan to either the Turkish or Saudi Arabian league. I won't call him a bad goalkeeper because I saw him play for Inter Milan. I would rather say he is low on morale currently. This is the main reason why he needs a new environment.
I also agree with you that Onana needs a new environment because it's clear as day that Man UTD don't hold him in high regard anymore. It now looks as if he is an average goalkeeper whereas he was undoubtedly the best goalkeeper you could have. When he was at Ajax, he was fire, at Inter Milan, you can't dare him but as he moved to England, his form and confidence dropped. I feel that he'd be appreciated somewhere else more than he is now at Man UTD.