What can you do if he's under contract?
Oh nothing other than try and sell him. The issue is that we haven't done that for a very long time. Luke Shaw has been injury prone for years at this point and I have never heard talk of moving him on. Martial had a lot of issues with injuries but again, we never moved him on.
All you can do is get rid of players when they show that they are not capable of retaining the fitness required at the top level. Other leagues are slower and less physical than the PL and other teams are willing to take quality players with injury issues because of the quality they can offer even if its not all the time. Any club with ambitions of winning things can't afford to carry players who are only available half the time unless they are utterly world class.