There would be no way I start the season(if I was Kubiak and assuming he realizes his a$$ could be on the line) without signing another RB.
JJ and SS both had MAJOR injuries last season and C. Henry is still a ??. I would start combing all the preseason games and see what teams release what RB's. My early favorite would be Keiland Williams out of LSU(yes, another SEC RB) signed as a UDFA, currently playing for the Redskins. I think we have a few connections on that team.