#3 Luke Shaw
Once an up-and-comer regarded as the best young left-back in England, Luke Shaw's development has been stunted over the past couple of seasons. Injury has been the main problem, but his performances since returning from his time out haven't been great, either. It's naturally led to many fans questioning his role at the club and whether he's really deserving of a first-team place at all next season; after all, there's plenty of better options out there.
Perhaps United have simply kept him on the books to meet the Homegrown players requirement, and given how expensive English players are in general, they've probably opted to stick with him rather than gamble and purchase a replacement who could be even worse. This would make sense if it were the case, but if it isn't and they just aren't sure what to do with him, the best thing to do right now would be to offload him for as much money as they can get.
From there, invest heavily in a replacement, someone with the calibre of Ben Chilwell, who is homegrown, and work with whatever that brings.