I think the general worry with him is not that he's having particularly bad games: it's that he's regressed.
In the first half of last season, he was everywhere. He was making the runs, he had the intelligence to put nice passes (the one to Joe Cole against West Ham comes to mind), he was a decent finisher (and even chipped Mignolet), he was taking advantage of his pace and correctly using turns of speed.
He's not showing anything of that right now (Notts goal notwithstanding). When he comes on games he doesn't offer much (hardly "impacts"), outside of giving away some fouls here and there. People here have rightly given him "credit" for "taking the sting out of the game", but that's what you expect a defensive sub to do, not the magical Sterling.
It's like he had lost a bit of his general "vision", of where to be and what to do to make things happen. Inconsistency is of course a part of youngsters, but that's usually a barometer when a youngster perfoms one week yes and not the next, or when he plays a bit sub-par but showing the flashes.
He's not doing that and people may be having a sense of "where's the bits or even flashes of the player we saw?"
IMO, he's still absurdly young, so of course, he has all the time of the world to develop. I personally think he should do that here or in a PL side; our loans to lower league teams haven't really resulted in anything. He can still be a fantastic player, but it's just a question of where did that old Sterling went, and whether he can find that in himself again.