Runningback is one position where rookies can contribute right away. Williams might not be the most dynamic prospect out there, but he seems pretty solid overall. I wouldn't be surprised if he's playing as much or More than Monty by the end of the year. Packers have had recent success with rookie runningbacks (Lacy, Starks in the playoffs, Brandon Jackson). If it appears at the end of training camp that the rookies aren't ready, there will be veterans available to be picked up.