RENTON, Wash. - Already thin at cornerback, Seattle lost second-year starter Walter Thurmond to a broken fibula and serious ankle injury, coach Pete Carroll said Monday.

Thurmond was placed on injured reserve, joining veteran Marcus Trufant, whom Thurmond replaced in the starting lineup after Trufant was lost with a back injury before Week 5.

Thurmond suffered the fracture when he broke up a pass in the second quarter of Sunday's 6-3 loss to Cleveland and was landed on. He also suffered a high ankle sprain, but the extent of that injury wasn't known until Monday when it was found that Thurmond suffered significant ligament damaged that will require surgery later this week.

The expected recovery time on Thurmond is four to six months. Rookie Richard Sherman will move into the starting lineup in Thurmond's spot.

"It's legitimately a long time of recovery. ... This is going to be a long haul," Carroll said.

The second-year cornerback out of Oregon missed most of his senior year with the Ducks after suffering a major knee injury. Sunday's game in Cleveland was his fourth NFL start. Seattle claimed rookie defensive back Ron Parker off waivers on Monday to take Thurmond's roster spot. Parker was with Seattle during training camp.

Thurmond's was the only significant injury suffered during the loss for Seattle. The other major injury happened during pregame warmups when lower back spasms locked up running back Marshawn Lynch.

Carroll said the back issue is one the Seahawks were aware of soon after Lynch arrived last year in a trade with Buffalo and has been kept under control. He's not sure what caused Lynch to suffer spasms to the point where he couldn't play.

"We were surprised by that, that it popped up and became such an issue," Carroll said. "We're going to have to be careful how we project this right now because he was OK going into game day. He had some symptoms but this kind of jumped at us. We'll have to be careful with him through the week."

Quarterback Tarvaris Jackson, tight end Zach Miller and center Max Unger are the other unknowns for Seattle, who host Cincinnati this week. Jackson (pectoral) went through pregame warmups on Sunday but was inactive for the game and Carroll said it will be later in the week before they know if he can play against the Bengals.

Miller still has not been cleared from a concussion suffered against the Giants on Oct. 9, while a foot injury kept Unger out against the Browns.