Joe Russo admits to ComicBook.com there's really no such thing as truly "permanent" when it comes to this sort of thing in Marvel movies. In a world of magic and super advanced science, the possibility for Hulk being healed is certainly there, but the point is that Hulk will need to be actively healed in some way for this damage to go away, he's not simply going to heal if he puts some ointment on it and wraps it in a cast. Also, as Joe Russo puts it, for him the damage is permenant, because it's somebody else's job to figure out what happens next...