Mostert (undisclosed) is not participating in Monday's practice, Daniel Oyefusi of the Miami Herald reports.
Monday's practice is unofficial, so the Dolphins won't release an injury report to confirm Mostert's status, but it's possible he's simply benefiting from a veteran rest day. Mostert appeared fully healthy entering the team's bye, but with De'Von Achane (knee) having been designated for a return from IR, the backfield situation warrants close monitoring heading into Sunday's game against the Raiders. If both Mostert and Achane are cleared to play, they will stand to split time as effective co-starters.