My only issue with Delph is his ball retention under pressure, it is ok and not great, which I think is a major reason why Pep is open to him leaving. Plus the injuries.
He scored a cracker last season for us and i would have no issue with him staying if we are confident his injuries are not a concern. He drives the ball forward very well and he really hasn't had a fair chance to show what he can do, but that is not anyone's fault,
If he goes, good luck to him and someone is going to get a decent player who can be even better with a consistent run. But if he stays I won't be unhappy as I think he could really kick on, surely unless he's told he won't get even on the bench most games then he wants to stay also, given the quality he'd be alongside and the chance to play in Europe.