FLAGSTAFF, Ariz. — The Arizona Cardinals went shopping for centers today after injuries hit two of the three listed at the position on the team's 80-man roster.

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.

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.

"It was really just to clean it out and make sure we could try to alleviate the swelling and the soreness," Whisenhunt said.

The injuries left second-year pro Lyle Sendlein as the only experienced center in camp.

"This gives him a chance to really get the bulk of reps," Whisenhunt said. "We just have to be careful now that we don't wear him out."

Hercules Satele, an undrafted rookie out of Hawai'i originally was listed as a guard on Arizona's roster, is the backup center behind Sendlein on the depth chart.

Sendlein, an undrafted rookie free agent out of Texas a year ago, was thrust into action when Johnson went down in last year's season opener. He went on to make two starts and appear in 12 games overall.

"I feel good because I know all my plays, I feel like I can play my role on this team," Sendlein said after the second Friday practice. "Coming into camp I was expected to be a backup and I was comfortable with that. Being a starter, I did that last season, so it's all the same to me, really."

Matt Leinart said switching centers does affect the quarterback.

"You get used to a guy, whether it's Al or Lyle," Leinart said. "It's more mental or anything, but it's always nice to have the same guy."

But Leinart, who missed most of last season with a broken collarbone, knows that NFL players have to expect injuries. He said Sendlein will benefit from the situation.

"It's actually good work for him," Leinart said. "He played quite a bit last year. We need Al in there but I think Lyle provides a great backup. He has proven he can go in there and make the calls and be a good center."