I think you have to first accept that injuries can happen. At best you can minimise the risk, you can't prevent them. If someone gets an injury or a reoccurrence that doesn't mean someone must've failed to do their job properly.
Hamstring tears are a bastard, they often reoccur. All you can do is strengthen as much as possible, but it might well still go. Same with any player coming back from a long time out, they often suffer follow-up injuries in the weeks after they return. People with Tetteh's physique often suffer with muscle injuries particularly.
People keep saying that Rosenior blamed himself for Allahyar's second injury, which is sort of true but it was a bit of a throwaway comment as part of a bigger answer where he actually explained that Allahyar had met every metric to show he was ready to return, all his fitness levels were there. What more can we do? Maybe he wouldn't have played three games in a week if we had other wingers at the time, but even then, if all the data shows he's 100% fit then he should theoretically have been fine to play those games, but a muscle can go at any time, particularly one that's gone before and particularly on a fast player like Allahyar.
Of course injuries happen, but to shrug our shoulders and say the season we just had was bad luck is the naive thing to do, not the scrutinising of the medical staff I'm suggesting.