Mostert couldn't ready himself for game day as quickly as he usually does, a source told ESPN's Adam Schefter, and the Dolphins were not comfortable playing him against the Ravens.
Mostert leads the NFL with a franchise-record 18 rushing touchdowns and is fourth in the league with a career-high 1,012 rushing yards. He is tied with Christian McCaffrey for the league lead with 21 total touchdowns.
Mostert is one of several key Dolphins players officially listed as questionable. The 10-year veteran was limited in Friday's practice after not practicing Wednesday or Thursday.
The Dolphins (11-4) likely will turn to rookie De'Von Achane and veteran Jeff Wilson Jr. at running back against the Ravens (12-3), who boast the NFL's stingiest defense in terms of points allowed (16.3 per game).
The Dolphins, who can clinch the AFC East title with a victory over the Ravens or a loss Sunday by the Buffalo Bills (9-6), also will be without wide receiver Jaylen Waddle, who is out because of an ankle injury.
For the Ravens, they will not have two starters in their secondary against the Dolphins' top-ranked passing attack, as both safety Kyle Hamilton (knee) and cornerback Brandon Stephens (ankle) are inactive Sunday.