What exactly do you mean with the level he's on? It's difficult to say right now, since he barely plays, based on the past I'd say he's on the road to becoming one of the very best. There have been moments where I actually thought he already was one of the very best.
Throughout the qualifications with the Belgian NT, he was by far the best & most important player. I'd go as far as saying that without him, we would not have been qualified yet at this moment.
Hard worker, amazing range, tireless runner, good reader of the game, brilliant at decisive passes, tactically aware & a fairly good eye for the goal as well.
Can play in a variety of (attacking) mid roles, as well as the wing. Personally, I think he can even play as a 2nd striker, if he's allowed to drop back & doesn't need to stay too high up the pitch.
Outclassed basically everyone when he played in Belgium, and from what I've been told, he was one of the better players in the Bundesliga last season (but I personally don't watch that league).
Apparently, Mourinho does not like and/or rate him at all though. Which does raise some questions.
And there seems to be a clear pattern in his performances: if he gets to play all the time, he'll deliver. Whenever he's been out of the game for a while (either because of being benched or injured), he seems to have trouble getting back to the level he's capable of. He seems to be the perfect example of a player who just needs to play all the time to be able hold on to his form. Which obviously may be difficult at the bigger clubs.