Bay, who was hitting .240 with three home runs and five RBI in 15 games for New York this season, was diagnosed with a broken rib on his left side.
Meanwhile, the Mets said Pelfrey was idled retroactive to April 22 with swelling in his pitching elbow. The right-handed starter has posted a 2.29 ERA over three games this year.
Their roster spots will be taken by infielder Zach Lutz, recalled from Class AAA Buffalo, and left-handed hurler Robert Carson, who was toiling for Class AA Binghamton.
2014: The Year in Music [PHOTOS]