The move was made less than 24 hours after Boston outfielder Jeremy Hermida bruised his left forearm in a game against the Baltimore Orioles. Hermida also hurt his chest, although he said Saturday he could play if needed.
But Red Sox manager Terry Francona decided the team needed an extra outfielder, so Reddick was summoned and was placed in the starting lineup Saturday night against the Orioles.
Atchison has gone back and forth between Pawtucket and Boston this season. He's 0-1 with a 4.50 ERA in nine games.
"He has options, he knows it and I think he understands," Red Sox manager Terry Francona said of Atchison. "I think he gets frustrated, he also knows we think a lot of him as a pitcher, and sometimes guys get on that shuttle when the team gets in a little bit of a bind like we potentially were.
"We needed a healthy outfielder [and] unfortunately he caught the bad end of it."
Reddick, conversely, was on the positive end of the transaction.
"Well, I don't really expect much of anything. Just come up here and try to help them as much as I can, whether it be short term or long term," Reddick said. "My numbers weren't really showing much, but the last two or three weeks I've been hitting the ball really well, having great at-bats."
Reddick was batting .191 at Pawtucket through Friday.