We're such a role and substitution heavy defense, a guy like Pettway could make an impact even if he's only a situational player. His departure clearly opens up a sub-rusher spot for Anderson and Allen. It can be an impact position. Upshaw had a decent year as situational rusher in 2009 and came up huge with his scoop up fumble recovery in the national title. Dareus had a huge season in 2009 when he was mostly a substitution player after Deaderick came back from his gunshot wound.
I think that "first" sub-rusher, nickel back, and dime back are key cogs of the defense even if they aren't listed starters. There is a lot of playmaking opportunities to be had at those positions.