Mostert (knee) practiced in a limited capacity Thursday, Keiana Martin of the 49ers' official site reports.
Mostert appears to be testing out his sprained MCL this week, having his practice reps capped in the process. Friday's injury report will be telling for his ability to return Week 5. If Mostert is unable to suit up and misses a third straight game, Jerick McKinnon, Jeff Wilson and JaMycal Hasty will be the running backs available to the 49ers offense.