Aggravated shoulder Sunday night

Coach Kyle Shanahan said Monday that Mason aggravated his existing shoulder injury in Sunday night's win over the Cowboys and is viewed as day-to-day, Nick Wagoner of ESPN.com reports.

After Sunday night's win, Shanahan said Mason could have returned to the game after he went to the locker room just before halftime. Mason, however, never played another snap, as Isaac Guerendo handled backfield duties the rest of the way. With Christian McCaffrey (IR, Achilles/calf) expected back after the Week 9 bye, Mason's days as San Francisco's lead back appear to be either numbered or over.