SANTA CLARA Nose tackle Ian Williams is out for the season after having surgery Monday to repair a broken left ankle, 49ers coach Jim Harbaugh said.
That puts Glenn Dorsey in a starting role on the defensive line and sends the team in search of a backup nose tackle, difficult at this stage of the season. Harbaugh said the 49ers will make a decision on the position in the next day or two.
Williams, who made his first start Sunday, was hit low by Seahawks right guard J.R. Sweezy while engaged with center Max Unger. That sort of "cut block" is legal in the NFL, but it drew the wrath of Williams' defensive teammates after the game.
Dorsey, who signed with the 49ers as a free agent in the offseason, had five tackles and one of their four sacks of Russell Wilson.
"He played well," Harbaugh said of the six-year veteran. "Had a lot of good snaps."
The 49ers had several potential backups at nose tackle, but two of them, Lamar Divens (hip) and Will Tukuafu (knee), were released with injuries and cannot be re-signed until later this season. Another, rookie Quinton Dial, is on the non-football injury list and can't play until Week 7 at the earliest.
One possibility would be signing undrafted rookie Mike Purcell from the practice squad. The 49ers will have an open roster spot when they place Williams on injured reserve.
Another injured player, rookie safety Eric Reid, was asymptomatic after suffering a concussion following a hit on Seahawks wide receiver Sidney Rice. Reid still must go through the league-mandated protocol before returning to practice.
Harbaugh said tight end Vernon Davis (hamstring) and defensive end Ray McDonald (ankle) were being examined Monday.
X-rays taken Sunday night showed McDonald's ankle was not broken.
"Nothing to report there," Harbaugh said of McDonald.
Read Matthew Barrows' blogs at www.sacbee.com/sf49ers and listen for his reports Tuesdays on ESPN Radio 1320.