My question is, from his perspective.
Secondly, he broke his leg while he played in a match for Manchester United, this club. I would say, it is the club's responsibility, and obligation to stuck with him, stand by him. He did not get into injury from speed driving, or drinking, or having haircut, or in a training session. His leg was broken during a match for Manchester United.
It's the clubs obligation to honor the contract & pay what he's owed and help him recover. It's not the clubs obligation to not replace his role if he doesn't seem capable of filling it and losing out on quality on the field because of it.
I understand you're playing Shaw's advocate here, but from my perspective the club always comes first, the individuals in second-line. As such I want to give Shaw the chance to succeed and stake a claim to the spot (a bonus because he's English and that matters for the home-grown quotas), but I don't want the club to be negligible with the quality in our squad just for that to happen. If Shaw can't accept that the club will put their interest and quality before him, then he should find another club.
The same goes for my personal reasons for wanting some of our center-backs gone. It's only down to wanting better quality at the club. If they are English, all the better. If the English aren't up for it, then they should be looked past for prominent roles in the first eleven.
Although I don't think there is a big quality difference between Jones, Smalling and Rojo so won't touch the debate too much. (leaving Blind out as he's a lesser CB to the other three)
Mind, this is from my perspective as a fan.
If I was English I'd probably be of the mindset that a few sub-par English players in the first eleven would be good for the national team and use that as a argument to not replace them.
If players in general showed as much loyalty to their clubs as some club-legends did, I wouldn't mind sticking by them, like how we gave Fletcher all the time in the world without really replacing him in the squad. Or how you give older players (Carrick now for example, although he seems to have a added benefit on the squad dynamic) the chance to stay even though they are quality-wise overdue a club-change.
If City, Real, Bayern etc can keep their players happy with having doubled up on certain position of high quality, then we should be fully able of doing the same. Especially as we seem to pay extraordinary amounts of money for squad-players.