Starter Al Johnson underwent arthroscopic surgery on his left knee earlier in the day. Coach Ken Whisenhunt said he hopes to have Johnson back by the season opener Sept. 7 at San Francisco.

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But reserve guard-center Scott Peters went down with a much more serious injury to his left knee. Peters, a third-year pro out of Arizona State, was carted off the field at practice on Thursday. The team placed Peters on injured reserve Friday, meaning he is out for the season.

Peters, a fourth-round draft pick by Philadelphia in 2002, has been plagued by injuries throughout his pro career. He spent last season on the Cardinals' practice squad.

To help fill the void, the team claimed center Pat Ross off waivers from the Carolina Panthers.

The 6-foot, 300-pound Ross, who played for Boston College, has bounced around various training camps and practice squads since signing with Seattle as an undrafted rookie in 2006. He was released by Carolina on July 31.

Johnson missed two games last year because of problems with the same knee. Whisenhunt said the decision to go ahead with operation on Friday was made to head off any lingering problems this year.