Simply watch him over the last 4-5 games, his decline in play is pretty evident despite what his tackle stats say. He started missing tackles (Rainey 80yd TD run), had trouble shedding blocks, and picked some poor lanes and got swept up in the blocking far too often (the NE debacle).
I sort of expected this. He is listed at 238, and I think that's pretty generous. You have to remember he's coming from playing 12 games in college to running every snap over a 16-game NFL season. There's really no way to prepare your body for the brutality of that at his position, so I think he wore down both mentally and physically. I bet he struggled to maintain 230 by the final week.
I think a full off-season with our training staff is going to do wonders for him, and I am very excited to see how he performs next year. I bet he starts next season at 245 or so.