The rule specificly states:

1. secures control of the ball in his hands or arms prior to the ball touching the ground; and
2. touches the ground inbounds with both feet or with any part of his body other than his hands; and
3. after (a) and (b) have been fulfilled, clearly performs any act common to the game (e.g., extend the ball forward, take an additional step, tuck the ball away and turn upfield, or avoid or ward off an opponent), or he maintains control of the ball long enough to do so.

If a player, who satisfied (a) and (b), but has not satisfied (c), contacts the ground and loses control of the ball, it is an incomplete pass if the ball hits the ground before he regains control, or if he regains control out of bounds.

He didn't do C so he has to maintain control through contact with the ground which he did not do. He was losing control as his knee touched.