The 49ers placed linebacker Parys Haralson on injured reserve and signed 13th-year veteran Clark Haggans.
Haralson's injury was not disclosed, but he did appear to have an issue with his left arm in the third preseason game.
Haggans was released by division-rival Arizona on Friday.
However, it's uncertain whether he will be eligible to play in the 49ers' season opener on Sunday in Green Bay. Haggans is facing a three-game suspension as a result of DUI conviction in June and is scheduled to have a hearing with the NFL on Wednesday, according to the Sacramento Bee.
Haggans, 35, started 16 games for the Cardinals last season, when he had 3.0 sacks. He has started 104 NFL games in his career and has 46.5 sacks.
Haralson has 295 tackles and 21.5 sacks in 86 career regular-season games.