(White Sulphur Springs, WV)  --  The San Francisco 49ers are losing their starting running back for the season.  Raheem Mostert announced on his social media Tuesday that he will undergo season-ending knee surgery and miss the 2021 season.  San Francisco originally estimated the running back would miss eight weeks with "chipped cartilage" after Mostert got injured in the first quarter of the team's season opener.  Mostert missed eight games last season with knee and ankle injuries and is in the final season of his three-year contract.