You have a point about City's weak defense, but if Donnarumma plays on Sunday, I expect him to make lots of saves, so I'd go with a draw.
Why the so much hype on Donnarumma?! He conceded four goals against Isreal? This is a game I expect Manchester United to win because, a lot of Manchester United players are available but I can't say the same for Manchester City. This might sound funny but Ruben Amorim remains the only manager yet to be defeated by Pep Guardiola. I don't expect a draw from this fixture, it has to be a win for Manchester United.