I wouldn't say he isn't playing well with Mata. His general play is good. I think what is frustrating most of us is his lack of a killer instinct in front of goal. The amount of times he has been put in and he fails to get a shot off is not what you expect from someone of his talent. Players will miss chances but he should have really scored another 3-4 goals this year already and that will cost us in the long run.
Don't know if it will cost you though, looking very good at the moment. How you deal with the big games and after losing a game will be the Big tests for you.
If I was a betting man I'd have a pop at you winning the league.