His stats are down a bit in December, but at least part of that is due to less minutes as well. Of the 8 games so far this month, he's played less than 30 minutes in 4 of them. His stat line didn't look pretty tonight, but it was in 24 minutes. If he plays 35 minutes, he's up around 19 points and 13 boards, which is about what we expect.
Plus, it's really just his points & rebounds that are down (he's actually shooting a higher percentage from the field, a higher percentage from the line, his assists are up, and his turnovers are half what they were in november), and of those two, the only one that should raise an eyebrow is the rebounds (although certainly no need to panic over that yet).