Where did you here that? Cook would be a damn good pick if indeed we go for an RB. He's basically a Montgomery type speed back - the best of that type IMO in the draft. That still doesn't - and damn well shouldn't - run 50 times a game or anywhere near it. It just means having an even more effective change of pace to Aaron Rodgers passing first and most.
The way D enters into it is you score and score and score some more, and force the opponent to take some chances. Then you get ball hawkish and come up with turnovers.