The problem United will face is an identity crisis. Can they realistically transition to a footballing club from Ferguson's very effective brand of kick and rush?
Not with David Moyes, that goes without saying.
Even beyond that it looks fairly grim- Fellaini, Van Persie, Valencia, Young etc their latest big money buys are certainly not going to slot into that system. Their big hopes like Welbeck, Jones, Cleverley aren't of that brand. Maybe Jones could slot in at the back and Cleverley/Welbeck play bit part roles.
Realistically the defence could manage but they're going to need to buy an entirely new attacking line up to fit around Januzaj and Mata. Neither of which is a particularly pure footballer. That's going to be a very painful transition to make and will take more than 2 seasons at least. They could very well not get back into the CL next year, although I suspect Liverpool will struggle- they've only played 39 games this season to City's 52 and Chelsea's 51.
Similarly Arsenal will have to transition from Wenger's style, which will admittedly be far easier by just bringing in a continental manager, but there will be teething problems.
Imo, right now, the next few season's are City and Chelsea's, and from then on it's anyone's game but Arsenal, United and Liverpool hold financial dominance. It's a very very exciting time for PL.