One thing to consider: besides DeSean Jackson, McKelvin is the best returner in the draft and he will be part of the active roster just by his playing on special teams. I am not sure if Rogers-Cromartie will be capable too, but Mike Detillier is very high on him also as a returner.
Whatever the defensive position we take, we only can expect a player to contribute in the rotation and get the starting position on year 2 or 3.
The only rookie that I can see coming and making impact from day one is Branden Albert as a left guard. If the saints want the first rounder to provide results immediately, then Albert is their guy.