What job could he possibly be holding out for? If any manager gets offered the United job they should take the honor as there's not many other bigger teams and he's already managed Real. The only thing he may be concerned about is his own reputation. It's much easier coming in at the start of the season where you can build your own squad, buy the players you want and bond with the current squad. At the moment he has to turn around an under-performing United and that won't be easy and what happens if they finish badly at the end. Will he then be disgraced and people calling for his sacking?
He probably wont be, but he's probably the only option they have right now without getting someone arguably worse than Mou or who is seen as a step-down. No point firing Jose to get someone who's not even as good as him. New managers rarely come in and turn the team around straight away though either. When teams are playing poorly or not to expectations they just need a fall guy and someone to blame for the issues and the managers are put on the chopping block as it's easy to blame one person than the entire team. I don't think Jose is doing anything tactically inept. Sure, there are always different formations and tactics to try, but he's won plenty of titles before playing essentially the same or similar way.