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.
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"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."
Grubbs had a procedure on the ankle in 2009 as well, but has been remarkably resilient. He started 60 of the 64 games he has played since being drafted in the first round in 2007. The 64-game streak is the team's fourth-longest active streak.
Thin at wide receiver
Injuries have depleted the Ravens at wide receiver enough that in one drill Tuesday, Justin Harper and Brandon Jones worked with the first team. Harper spent the past two years on the practice squad, and Jones, a five-year NFL veteran, did not play in 2010.
Sidelined were second-round pick Torrey Smith and veteran James Hardy, both with apparent hamstring tightness. Anquan Boldin is the only receiver in camp who played for the Ravens last season, and he takes his normal turns.
Cameron said he hopes both Smith and Hardy can return to practice Wednesday.
Reid settles in
Cameron said rookie offensive tackle Jah Reid had his best day of practice after working through the predictable early transition.
"All the [young] linemen were overwhelmed, because it is, it's overwhelming," Cameron said. "But I think [Reid] settled in today. I saw a little difference. Things weren't as confusing. It looks like he corrected some mistakes from [Monday], and that is a great sign when young guys start correcting mistakes this early on."
Also sitting out Tuesday's practice were cornerbacks Jimmy Smith (groin) and Domonique Foxworth (knee). … Undrafted rookie wide receiver Hakeem Moore (Towson) hasn't worked since Friday because of an apparent injury. ... The Ravens waived undrafted rookie offensive tackle Alex Solot. … Kicker Billy Cundiff hit field goals from 32, 39, 50 and 59 yards before missing from 45 — his first miss of camp. … The Ravens were 1-for-2 in goal-line offense. Starting from the 3-yard line, Ray Rice powered into the end zone in two carries, but rookie Anthony Allen was unable to score in three tries from the 3. … Safety Ed Reed wore Matt Stover's No. 3 instead of his own No. 20.