The Raiders announced Wednesday that strong safety Mike Davis, who left in the fourth quarter of Sunday's loss in San Diego, underwent arthroscopic surgery to have torn cartilage removed from his left knee Tuesday.
Davis was put on the injured-reserve list, which means he must miss four of the regular season's six remaining weeks.
Coach Tom Flores, asked about the seriousness of the injury to Davis, said: "It was just a 'scope. He's going to have to miss four games, but you usually can come back from 'scopes. Plus, Mike's a tough guy."
Davis' spot will be taken by Stacey Toran, who filled in for Vann McElroy at free safety the last two weeks. McElroy has had a hamstring pull, but the Raiders expect him to play Sunday.
To take Davis' spot on the roster, the Raiders signed free agent Ricky Williams, who was cut in the last two training camps.