Casey Rabach, on July 26. (Luis Alvarez / AP)

Rabach, who is 33 and underwent shoulder surgery in January, received a two-year contract. The Ravens were in a position of need after Chris Chester signed with the Redskins.

Rabach was a third-round pick by the Ravens in 2001 and became a full-time starter in 2004. Birk, 35, has been the Ravens' starting center the last two seasons, but has had to have his knee drained at least once a week in order to get through 18 games. ESPN reports that Birk may be having arthroscopic surgery soon.