To take Reddick's place on the roster, the Red Sox summoned veteran left-hander Billy Traber from Triple-A Pawtucket.
Reddick, who had only advanced as far as Double-A Portland before going to the Red Sox on Aug. 1, was optioned to Pawtucket.
To make room for Traber on the 40-man roster, the Red Sox designated infielder Gil Velazquez for assignment.
Having gone through his entire bullpen in Tuesday's 13-inning loss, manager Terry Francona badly needed another arm at his disposal for Wednesday's game. Along with Traber, Francona said he would also have starter Clay Buchholz available in an emergency.
Traber, 29, has pitched 32 games this season, four of them starts. He is 7-5 with a 3.32 ERA. Traber has pitched 95 games in the Major Leagues, going 12-14 with a 5.54 ERA.
"They've got a pretty good wealth of arms down there, so the fact that I got called was pretty much a total shock," said Traber.
As for Reddick, he played four games for the Sox, belting a homer and two doubles while hitting .313 in 16 at-bats.
"He took a lot of, probably nervous energy, which I'm sure he had, and transformed it to a guy that helped us win some games," said Francona. "I was proud of him. We all were. He's got a lot to learn. He's a young kid. But he came from Double-A, had all that nervousness and he took some pretty good swings.
"He was aggressive, he was energetic. Guys liked him. There's a lot to like about him. He's just a nice country friendly kid that likes to play baseball. He's got some thunder in his bat. It's another exciting kid that hopefully is coming through that can help us win."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.