From the regular camera angle on the play, i'm guessing either his knee or his lower leg bent the wrong way on the tackle. You see Fitz go into his legs and too many bodies to see what happens but then his legs are up in the air a split second later. Maybe a Willis McGahee situation? If he's lucky hopefully it's just a clean break of a lower leg bone. I assume anything else it could possibly be would be season ender/long rehab