But don't expect the Ravens to go after Olin Kreutz or Shaun O'Hara.
Birk, 35, had surgery Tuesday to clean out the bursa sac in his left knee. If Birk didn't have the surgery now, it likely would have swollen up and bothered him the entire season.
"We decided to go ahead and get that cleaned up right now," Harbaugh said.
In a typical offseason, Birk might have had the surgery in February, March or April. But the Ravens weren't able to check him out until last week.
"He wasn't here, so you don't know what a guy looks like," Harbaugh said. "That's probably one of the negatives of having this whole situation."
Birk, the Ravens' starting center the past two seasons, hasn't missed a game since 2005. This isn't the first time he's had knee issues. In order for Birk to play 18 games this past season, his swollen knee had to be drained at least once a week.
The Ravens wanted to add Rabach for depth. But the former Ravens' third-round pick failed his physical because he hadn't recovered from offseason shoulder surgery.
One backup free-agent center remains on the market: Rudy Niswanger from Kansas City.
Undrafted rookie Ryan Bartholomew (Syracuse) practiced with the first team for the past two days, and Bryan Mattison could take over when he returns to practice Thursday.
If the Ravens are truly in a pinch, they could turn to Marshal Yanda, the versatile lineman who was the team's third-string center last year.
Not looking yet
There was no panic at Ravens headquarters after quarterback Marc Bulger announced his retirement Wednesday. Harbaugh had already estimated there was a 50 percent chance of that happening after the season.
So, will the Ravens jump on an experienced backup like Brodie Croyle, Jake Delhomme or Todd Bouman?
Based on Harbaugh's comments, it doesn't sound like it.
"This year, there are not as many guys available because of the late stage," Harbaugh said. "But there are a few veterans. I'm also confident that, in a week or two, there will be some veterans available if we decide to go that route. Right now, we want Tyrod [Taylor, rookie sixth-round pick] to get the reps and see how he does."
The three quarterbacks on the Ravens' roster are Joe Flacco, Taylor and Hunter Cantwell, a practice squad player from last season.
No trade for Umenyiora