The 49ers signed a veteran defensive back, Antone Exum Jr., on Tuesday and placed tackle Garry Gilliam on injured reserve.
Exum, 26, missed the 2016 season with the Minnesota Vikings with a leg injury. He played in 27 games the previous two seasons with two starts at safety. He also has background playing nickel back and has been a special teams contributor.
The 49ers lost both starting safeties, Jimmie Ward and Jaquiski Tartt, to broken arms in the last two weeks. Eric Reid has taken over at strong safety while rookie Adrian Colbert is the new free safety. Dexter McCoil is serving as the team's third safety.
Gilliam suffered an MCL sprain during his lone start of the season, Oct. 29 in Philadelphia. He was expected to miss 6-8 weeks and the 49ers had hoped he could return to the lineup in mid December. However, with so many other injuries they could not afford to keep him on the 53-man roster.
The 49ers placed Ward (arm) and receiver Pierre Garcon (neck) on injured reserve last week and safety Tartt (arm) and tight end Cole Hikutini on injured reserve this week.