Donald will need to really maintain gap integrity against a guy like Demarco Murray and that zone rushing attack. Runners like Murray eat guys that just shoot the gap upfield every play for breakfast, and that is Donald's number one attribute right now.
Experienced NFL linemen crash that d tackle down with a center and the pulling guard leads through the hole to get a hat on the filling linebacker, rinse and repeat. If Donald or Brockers or really whomever can't plug a gap and maintain integrity on running plays, it is going to be a field day (again) for Murray in St. Louis.