He cost us the most money and takes home the most wages. The expectation is rightfully higher for him.
He has decent to good stats for a midfielder, but they're less impressive when you factor in penalties. He also has brought an absolute circus to our door and is not without fault on the pitch.
As with Martial, I think he is capable of being better than he is and that is why i criticise him, along with what I've said above. So yes he's not the worst player in terms of performances, though sometimes (to often) he is one of them. When you factor all the good and the bad in, I don't think it looks favorable on him. This is coming from somebody that wanted him back here and defended him for a long time.