The 49ers are expected to place Mason on injured reserve this week after he suffered a high-ankle sprain late in Sunday's 35-10 loss to the Bills, Nick Wagoner of ESPN.com reports.
After Christian McCaffrey suffered a PCL injury to his right knee in the first half, Mason took on a larger offensive role versus Buffalo, finishing with 13 carries for 78 yards. Though he would have been in position to lead the San Francisco backfield for the rest of the season with McCaffrey's injury expected to require a six-week recovery, Mason is instead on track to join McCaffrey and IR and won't be eligible for activation until Week 18. With Mason and McCaffrey set to miss time, rookie Isaac Guerendo is now in line to take over as the 49ers' lead back beginning with this Sunday's game against the Bears. Patrick Taylor likely will be promoted from the practice squad to serve as the Niners' backup running back over the next month.