The Rams have no star, no sure fire lead pony, at RB. So, the only thing to do is to let it sort itself out by playing the first 3 guys on the depth chart to see if any of them can separate themselves from the others. It's a great opportunity for them.
It will also likely be that what each does best will slowly emerge, and perhaps the game situation and what the Rams as a team do best on offense will have much to say who gets the most snaps.
This will be somewhat experimental, so I expect there to be some glitches and false starts among the RBs and McVay's play calling for the first 3 or 4 games. Things will likely shift around during the progression of the season.
