Right now we have 3 undersized (from a run-blocking standpoint) receivers on the roster. Moss is not a blocker, while Williams and Manningham are too small for the task. This leaves only Crabtree as a blocker. This would be OK if one of these undersized WRs could return punts, but since Moss won't, Manningham never has, and Williams... well, do I really need to go further with that?
This means we must bring in another CB or WR fro the task. Based on the receivers' natural ability to catch the ball better than a CB (or else the CB would be a receiver as they say), it is much easier to find a WR for the job, and this player will almost certainly be on the smaller side for the position.
This means we are likely to have 4 non-blocking receivers, and Crabtree as the only exception. Even if we do bring in another rookie with size, chances are he won't find the field very often, buried behind Crabtree, Moss, and Manningham on the depth chart. This worries me as a run first team.