Atchison has made the Red Sox' Opening Day roster after an impressive spring, manager Terry Francona said Thursday.
The 34-year-old Atchison, who last played in the majors in 2007 when he threw in 22 games for San Francisco, was 1-0 with a 1.50 ERA in 10 games this spring.
He pitched for the last two seasons with the Hanshin Tigers of the Japanese Central League before signing a free-agent contract with the Sox in December.
Francona said the club was high on Atchison from the start.
"Strike-throwing ability, three pitches, changes speeds," Francona said when asked what impressed him of the right-hander. "His regular season started about a month ago and he knew it, but he did a good job. He attacked the strike zone with all his pitches. I don't think we see that changing.
"Right from the get-go, he was on our radar. He just pitched so well."
In 12 Grapefruit League innings, Atchison yielded nine hits, walked two and struck out six.
The decision on Atchison nearly finalizes the bullpen, but Francona said the team is still weighing other options.
Righty Joe Nelson remains "deep in the mix" for a bullpen spot as well and will travel to Washington with the club for Saturday's game before his final destination is determined.
Alan Embree, signed to a minor league contract in mid-March, will be given "more time to pitch" in either extended spring training or at one of the minor league affiliates. Embree has given up 11 runs — 10 earned — in just 2 1/3 innings.
Scott Schoeneweis, signed earlier in the week, is scheduled to throw in Thursday's game against Minnesota at City of Palms Park. He has pitched in just 1 2/3 innings thus far for Boston.