He needs to start building his managerial career now, and in my opinion only (before people start taking my thoughts out of context) - it should be away from United. He needs to start implementing his philosophy and gaining much needed experience. Most importantly, he needs to leave his comfort zone and challenge himself. If he doesn't, it will reinforce the idea that some fans have that he's just waiting for the job to be given to him, instead of earning it.
He also needs to leave for the good of the club because he's a distraction - a bad one. If he weren't, we wouldn't be discussing him so much. All the constant speculation about his future does not help the manager or players. I would venture, that any new manager that comes in will find this extremely uncomfortable, knowing the moment things go wrong, the press will start speculating that your assistant will probably be getting your job.