SANTA CLARA -- Nose tackle Ian Williams sustained a season-ending broken left ankle in the 49ers' loss Sunday night to the Seattle Seahawks, 49ers coach Jim Harbaugh said.
"It looks like he's going to be out for the year," Harbaugh said. "It's disappointing, and he'll have surgery today."
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Williams was injured on a legal cut block from behind by Seahawks guard J.R. Sweezy.
Veteran Glenn Dorsey takes over as the 49ers' starting nose tackle. He is the only nose tackle on the team's 53-man roster. The 49ers are not allowed to bring back Will Tukuafu or Lamar Divens because they were waived/injured recently. Rookie nose Mike Purcell is on the 49ers' practice squad.
"We're looking through that right now, and we'll make a decision in the next day or two," Harbaugh said of an expected roster move. Williams will go on injured reserve, making him ineligible to play for the remainder of the season.
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Meanwhile, rookie free safety Eric Reid is not experiencing any concussion symptoms, Harbaugh said. Reid sustained a concussion Sunday night in the 49ers' 29-3 loss. He did not play in the second half. Reid must go through the NFL-mandated concussion protocol before he can be allowed to return to practice and play Sunday against the Indianapolis Colts.
Harbaugh did not have any injury updates on defensive lineman Ray McDonald or tight end Vernon Davis, both of whom were injured in the fourth quarter. X-rays on McDonald's ankle were negative, and Davis sustained a left hamstring strain. The 49ers will see how those players progress during the course of the practice week before determining their availability for Sunday's game.