I think he does better when he gets explicit instructions on his role in the game. Which players to mark, where and when to make runs, who to pass to, etc.
When he comes on as a sub he doesn't get the level of instructions and it's usually at the same time that the opposition is making changes too and he can't adapt to those changes on the fly.
He's shown that he's a capable player under Rafa and Howe, both of whom almost certainly give him that level of instruction, but was crap under Bruce and his Let's just kick it about style.
Either way, it's time for him to move on. He's not getting much game time now and doesn't look great when he does get on. If we ever want to really start to complete for the league we need to be much more ruthless and move on those players that aren't capable of keeping up any more.