Gerhart needs to become durable. Riggins wasn't continually coming off the field and/or missing games with "rib contusions," ankle problems, pulled hamstrings, etc., like Toby often has.
When you think of a White power back you think of Riggins, Csonka, Taylor, and all the way back to Grange and Nagurski, indestructible types. White running backs get so few chances that they can't afford to be nicked up all the time. I know Gerhart tried to play through a bad ankle earlier this year, to his detriment, but his tendency to get nagging injuries has worked against him. Hillis too for that matter -- he was healthy in his great 2010 season, but was injured as a rookie in '08 when all the other RBs went down in front of him with injuries and he kicked ass as the starter before getting hurt himself, and every season since '10 he's had hamstring issues, concussions and other injuries.