He seems like a player that came out at a good level early and played some of his best football, then stopped improving just as you expect a midfielder to up a level or two. Simeone was starting to rotate him a lot last season.
That said, his situation right now looks more like a player that isn't wanted at all/or needed in the first place tactically, or has checked out in training ( probably not though if he's said he wants to stay). You can't judge someone who has proven to be very solid at the level he has on so little game time, at least in terms of it being a "not good enough for any big english team" situation.