The decision was expected, as Miller started in the preseason and received first-team reps throughout training camp.
Neither the Dolphins nor Miller would confirm the move. But barring an injury in practice or any last-minute setbacks, Miller will remain with the first team for Sunday's opener against the Cleveland Browns.
"We are going to release our depth chart later this afternoon, but we still have a full week of preparation," Dolphins coach Joe Philbin said Monday. "We're not playing a game for six days, and we'll see how things go."
Miller is in line to replace 2012 starter Reggie Bush, who joined the Detroit Lions in free agency. Miller won the starting job over Daniel Thomas, who averaged just 2.7 yards per carry this preseason.
Miller, a fourth-round pick in the 2012 draft, led the Dolphins with 4.9 yards per carry last season in limited playing time, rushing for 250 yards and one touchdown.