You probably guessed this from your own knowledge of how knees are and aren't supposed to bend (Warning: photo is painful to look at, but maybe not quite as painful as Fox showing the replay six consecutive times), but the preliminary diagnosis indicates that all-pro linebacker NaVorro Bowman has a torn ACL. The least the officials could have done was let him have his fumble recovery.
The play had all the markings of a huge embarrassment for the NFL. On a third-and-goal in the fourth quarter, Jermaine Kearse fumbled near the goal line. Bowman scooped it up, even as his leg was pinned underneath Kearse. Inexplicably, the zebras didn't see it, and the Seahawks came out of the pile-on with the football, and the play was officially unreviewable. A Pyrrhic defeat for Bowman, despite his incredible ability to cradle the ball even as he screamed in pain.
Sure enough, a league source tells Pro Football Talk that the NFL's competition committee will examine the review rules in the offseason, and decide whether fumbles should come under replay's purview.
As of now, rule 184.108.40.206 of the NFL Rulebook states that
"Non-Reviewable Plays include..recovery of a loose ball that does not involve a boundary line or the end zone."
This is arbitrary, and seems pretty dumb: The point of replay is to get the call right, and a play determining who has possession of the ball seems important enough not to be exempt. The rule almost surely exists to avoid lengthy replays on every loose ball, which often end in scrums impenetrable to the camera. (Bad things happen down there—pinching, biting, gouging. The NFL probably doesn't want anyone paying closer to attention to them.) But on occasions like this, when replays clearly showed Bowman had possession and was down, why not let technology assure a correct call?
The Seahawks fumbled on the very next play, and thank goodness. Had they scored and put the game away, this blown call would have gone down as one of the more controversial screw-ups. As it is, the league gets a chance to patch a hole before it becomes a debacle (like how new playoff OT rules came into effect a year after Super Bowl XLIII nearly went to overtime). That's little consolation for Bowman.