That's not to say that he should be the starter, or that he won't be the first RB off the bench to replace DeMarco Murray, presumably on third downs, and in obvious passing situations, but I think the Cowboys brain trust sees Dunbar as playing an entirely different role from Murray's in the offense this season. I believe they envision Dunbar as a Darren Sproles type of player, who has a specific package of plays, both runs and passes, and the quickness to create match up nightmares for the defense if he draws one on one coverage. Those plays will be in the game plan no matter who the starting running back is.
That's why my interpretation of the Cowboy's running back depth chart wouldn't even include Dunbar, who is essentially playing a different position than Murray, Phillip Tanner, and rookie Joseph Randle. The way I see it, if Murray needs a break, or if he should get nicked up in a game, it would be Tanner, not Dunbar, who would take over Murray's workload. In essence, Tanner is Murray's true backup, but he would only fill in for him on a short term basis. The real beauty of the way the Cowboys have constructed their running backs corps is that each player has a separate and distinctive role on the team, and they are:
Starter – DeMarco Murray
Backup – Phillip Tanner
Third Down Back – Lance Dunbar
Understudy – Joseph Randle.
I believe if DeMarco Murray were to miss a few games, or worse, that Joseph Randle would become the starter, leaving Tanner and Dunbar in their familiar roles as backup, and third down/specialty back, creating minimal disruption to the offense. It's genius really, and insurance against the kind of offensive drop off the team experienced when Murray was unavailable last year, rendering the ground game ineffective, which placed far too much of the burden (and a target) on the back of Tony Romo. I'm sure the Cowboys have a lot of faith that DeMarco can put together a healthy year and live up to his amazing potential, but they've taken some very prudent steps to make sure they can still maintain a balanced attack even if Murray does go down for any amount of time, from one series to the whole season. Murray and Randle are very similar runners, and the rookie, like any good understudy, only has to watch closely, and learn how Murray plays the position, so that if / when he gets the opportunity, he'll be able to imitate him as closely as possible, in his own way. The only question would then be if Randle can be as effective as Murray as the featured back, but if the Cowboys didn't think he had that potential, they wouldn't have drafted him where they did.
I think at some points throughout the season, all of the Cowboys backs will be given an opportunity to shine, and contribute to the team's success. Hopefully they'll all make it through the year without any significant injuries, and if that happens, the complementary nature and the versatility of this young running backs corps could turn out to be a real strength for a team that had the least effective ground attack in its franchise history just last season.