keep rock. he is one of the fastest players on the team(he ran the fastest 40 time the year he was at the combine). his speed allows him to get downfield fast on special teams where he will play.
btw, his size(lack of height) helps him on goal line runs, he can get lower than the defenders.
edit: fastest running back in that draft.
http://archive.profootballweekly.com...sis_032802.asp