The 49ers will have nine days to rest up for their next game against the Arizona Cardinals, but they likely will be without at least three starting defenders despite the layoff.
One, defensive end Tank Carradine, suffered a high-ankle sprain late in Thursday's loss to the Rams. Carradine was scheduled to have an MRI Friday that would determine the severity of the sprain, but Shanahan said that injured reserve is a possibility, implying that Carradine is suspected to have suffered a Grade 2 or Grade 3 sprain.
Shanahan said that decision probably would be made Monday.
Linebacker Reuben Foster suffered the same injury in Week 1. He was on the sideline without a walking boot Thursday, a positive sign. Still, Shanahan said he'd be "shocked" if Foster were able to play against the Cardinals Oct. 1.
He said strong safety Eric Reid, who went down with a PCL strain Sunday, was behind Foster in his timetable for recovery.
Two more defenders, safety Jaquiski Tartt and backup linebacker Brock Coyle, are in the league's concussion protocol after leaving Thursday's game. So is fullback Kyle Juszczyk, who also suffered a neck injury.