Grealish needs to work on the aspects of his game that are lacking. Until he can improve his marking and defensive engine, he'll always be a liability/luxury this club can't afford.
Talk of him flourishing at another club are probably valid. That said, I don't think the likes of Mourinho would play him. Neither would the Ranieri types.
As an Irishman I was quite disappointed when he declared for England. Looks like I had nothing to worry about... yet. In fact, the player I was hoping he'd replace in a couple of years is currently one of the best CAM's in the Championship - Wes Hoolihan.