They will do something at the RB position. I have little doubt about that. Stephen mentioned more than once that the world champs added a RB late in the season and were just fine (he failed to mention that it was necessary in part because Ridley tore his ACL and was lost for the rest of the season and that Blount was cut for quitting on his team). I don't know that Dallas will wait until deep into the season but I think in general they are trying to emulate the Patriots and their approach to RB. Feels sort of like a "we are smart, you know, like the Patriots" kind of thing to me but whatever.
Doug Martin would be almost a best case scenario as far as I'm concerned and he hasn't exactly lit it up since his rookie year. He is still young and has some nice upside though, so I would love to see them bring him in. There will be decent RBs cut or traded for next to nothing because they are going to get cut. If they can get a proven guy to add to the stable and maybe push off one of the guys they have now with a modest upgrade I will feel a bit better about the position.