Pittsburgh also selected the contract of veteran right-hander Jose Contreras, recalled infielder Jordy Mercer from Triple-A Indianapolis and optioned right-handed reliever Jared Hughes to the same club.
The move with Walker is retroactive to April 27, and the Pirates are hopeful he will be ready to be activated May 12. He is hitting .253 with one home run and eight RBIs in 23 games.
Brandon Inge is expected to make the majority of starts at second base in Walker's absence.
Contreras, 41, was signed to a minor league contract as a free agent Feb. 23 and made six relief appearances in the Pirates' farm system, allowing three earned runs in seven innings for a 3.86 ERA. He pitched in 17 games for Philadelphia last season before undergoing Tommy John reconstructive elbow surgery on June 20.
In 26 games with Indianapolis, Mercer was hitting .333 with one homer and 19 RBIs.
Hughes was 1-2 with a 4.61 ERA in 13 games this season after leading National League rookies last season with 66 relief appearances.