The converted reliever was hit by a liner near the wrist on his 20th pitch of a BP session on Wednesday at the Yankees' minor league complex.
Since pitching April 18 at Triple-A Scranton/Wilkes-Barre, Prior has made just one minor league appearance because of a strained groin and back soreness. He expects to pitch soon in a rookie-level Gulf Coast League game.
Prior has struggled with shoulder injuries and made his previous major league appearance in August 2006. The 30-year old went 18-6 with a 2.43 ERA in 30 starts for the NL Central champion Chicago Cubs in 2003.
Also, backup infielder Ramiro Pena, who had an appendectomy on July 18, has resumed a running program.