United are in a bit of a quandry.
Ronaldo is a professional, but he is ultimately a liability to the club.
He has made it clear that he doesn't want to be there, but then the team is still set up to play through him. Ultimately, he is simply not the same player he once was. Plus United's attacking wings don't create enough chances. This means they will struggle to score goals this season.
You can't afford to bench him because it will be an issue in the locker room, he will likely undermine the coach and there will be the added external media pressure.
Best thing for both parties would be to get a team that's willing to pick him up. Chelsea are trying to sign Aubameyang right now. Would be on the phone trying to convince them to sign Ronaldo instead.