Marion wasn't the "starter" because they loved to save him for the 2nd half of games. They'd let that big OL and Julius Jones, then Felix Jones/Tashard Choice wear the D down through the first half. Then they'd unleash the Barbarian on that already tired D in the second half and watch him make his own holes. There was only one season where I remember them giving him significant snaps in the first half, and it didn't go as well as we had hoped.