The Ravens signed center Casey Rabach to a two-year contract Tuesday, gaining insurance in case Matt Birk's balky left knee leaves them short at the position.

According to an ESPN report, Birk, 35, needs arthroscopic surgery on the knee. Birk played through the pain a year ago, when he needed to have fluid drained almost every week but didn't miss a start.

Coach John Harbaugh wasn't available to address Birk's situation — it wasn't his day to meet with the media — but offensive coordinator Cam Cameron said Birk had an MRI on Tuesday morning.

"That's exciting," Cameron said of the Rabach signing. "That gives us some depth at center. We will see how things have turned out with Matt Birk."

Rabach, a third-round draft pick by the Ravens in 2001 out of Wisconsin, returns to the team after spending the past six years with the Washington Redskins. A 10-year veteran, Rabach turns 34 in three weeks and is coming off shoulder surgery in the offseason.

He started only one season for the Ravens (2004) after Mike Flynn was hurt, and when the team elected to stick with Flynn the following year, Rabach left for Washington as an unrestricted free agent.

Rabach eases the pressure at a position where the Ravens recently lost their backup, Chris Chester, to the Redskins last week in free agency.

Remarkably, he is the sixth center on the roster, although Bryan Mattison, an exclusive free agent, isn't allowed to practice with the team yet. The three other centers are undrafted rookies — Ryan Bartholomew of Syracuse, Tim Barnes of Missouri and Justin Boren of Ohio State.

When Birk missed practices Monday and Tuesday, Bartholomew and Barnes alternated at center with the first-team offense. Bartholomew acknowledged the odds are against the rookies.

"You can't worry about the numbers game," he said. "You've just got to do your thing and do the best. I'm an undrafted free agent. My chances are slim to begin with. So it's just me going out and proving that I should be here. If I'm not, hopefully, my tape's good enough that other teams will see it and pick me up. All I can worry about is myself right now."

Linebacker Terrell Suggs was happy to learn Rabach was coming back.

"When I first got here, he was here and he kind of toughened me up, slapped me around a little bit," Suggs said. "So now that he's in the later stages of his career, I look forward to doing it to him a little bit."

Kindle makes progress

The news continues to be good on linebacker Sergio Kindle, who missed last season with askull fracture after falling down steps shortly before training camp started. New defensive coordinator Chuck Pagano said there have been no restrictions with the pass rusher from Texas.

"I can tell you that he's picking things up," Pagano said. "He has not shied away from any contact. There has not been one time. … We don't talk about [it]; I don't think anybody should talk about this, that or the other. We're just coaching the kid. He's showing up every day, he's at every meeting, he's taking notes, he's learning.

"He's way ahead of where I thought he'd be. So, knock on wood, if he keeps progressing like he's progressing, we'll find a role for him, obviously."

Kindle has participated in three live practices since Friday without incident.

Grubbs on the mend

Left guard Ben Grubbs, who underwent ankle surgery in the offseason, went through his second practice Tuesday since coming off the team's physically-unable-to-perform list. He said he was still in the recovery stage with the ankle, but he didn't use surgery as an excuse.

"I don't know how much to attribute that to my surgery because every year you come back as an offensive lineman, there's going to be some rust that you have to knock off," he said. "And because we missed that whole offseason, it matters more. So every day, we just try to come out here and try to be better than the day before."