The guy who looks most likely to emerge as a true No. 1 RB the earliest is the Denver Broncos' Royce Freeman.
Not only did the third-round draft pick look more impressive than veteran Devontae Booker in the preseason opener with four carries for 38 yards (including a 23-yard touchdown), but ESPN Broncos reporter Jeff Legwold has continued to stress that, "He's just too big, too fast not to play, and he runs with vision."
We do think Booker could emerge in the passing game and turn into more of a third-down, change-of-pace back, perhaps once Freeman gets more established. As the story also said, rookie backs tend to be stuck in timeshares, at least early in the year.