According to Adam Beasley, the Dolphins today named Lamar Miller as their starting running back, according to the team depth chart.
Per the Miami Herald:
Dolphins depth chart lists Lamar Miller as the starter for Week 1.
Miller and Thomas were given somewhat equal carries through the first few weeks of the preseason, though Miller seemed to pull away by Week 3.
That lead many to believe that the Dolphins were doing nothing more than trying to motivate Miller by suggesting that Thomas could be named the starter.
Thomas was reportedly told he would be receiving goal line carries, though, those too ended up going to Miller. It’s likely Thomas will see some time, especially on third down given blocking abilities but Miller will be the Fins starter going into Week 1.
Clearly more talented than Thomas, it would take some significant struggles (or an injury, obviously) for Miller to be supplanted as the starting tailback