Every time Nastsic has started to look even half decent as a defender, as if the penny is dropping, he has done something to let himself, & us, down & it's back to square one.
If that mistake is made with Nastasic trying his hardest, & getting it wrong, then the manager is more likely to forgive him, at least while he's young. When it's a mistake made just because the player doesn't fight for the ball though, it's unlikely he is going to make it. Add that to the fact he's not fast, or dominant in the air, what exactly are you buying when you sign him ?
If someone can toughen him up, he might be ok, but it was clear from day one that he wasn't the player some on here were making him out to be.
Anyway, good luck to him in his career. I hope he learns.