I had to check for last season's stats to reply
There is something obviously wrong in Hamsik's game in the last 4 months. Last year he was the serie A leader in crossing. The player with the highest number of successful crosses. He was in top5 in one more categorie (corners won) and top 7 in another category (assists).
Compared to last year, he's consistent only in scoring goals. He scored many goals both last and this year. He scores a lot of goals for a midfielder (only Kaka was ahead of him both this and last year).
But even this year 80% of his goals (9 goals) and assists (4) came in the first 3 months (September, October and November).
One has to wonder what is the problem. Why was he ever present everywhere on the pitch, involved in everything, and why is he lately like a ghost that only knows how to pop up in the box and score (which is something you'd expect from Trez, not from Hamsik).
Last year his position on the pitch was a little different so that might explain the differences from last year and last 4 months. But i can't see why there is such a difference between his performances before December and his performances after December. He plays the same role all season long so the reason must be something else.
Maybe he wants to be sold and he's forcing Napoli's director to do it, but if that's what he's doing then it creates another problem because no-one wants such players in the team.