"We just wanted to get him here just in case we start getting into the bullpen," manager Terry Francona said.
Atchison, 35, was with Triple-A Pawtucket during the first-round playoff series with Lehigh and recorded two outs in relief in a Game 1 loss.
The 6-foot-2 righty has appeared in 10 games this season with the Red Sox, compiling a 4.42 ERA and one save. He last appeared in a game on Aug. 26, when he gave up one run in three innings in a 15-5 loss.
The long reliever can provide much-needed innings for an injured rotation leaning heavily on the bullpen.
"I think mostly I can keep everybody fresh," Atchison said. "Another arm this time of year always helps in this situation. You want to keep the main guys ... as fresh as they can be for the stretch run. Hopefully I can come in and pick up some innings and help them out."