Atchison's placement on the disabled list is retroactive to Saturday. Tazawa arrived in Boston's clubhouse at Tropicana Field on Sunday morning and was active for the club's series finale against the Rays.
Atchison leads all qualified Red Sox relievers with a 1.76 ERA and ranks second in the American League with 46 relief innings spanning 37 appearances. He threw two-thirds of an inning Friday against Tampa Bay and allowed an unearned run on one hit with one strikeout.
This is Tazawa's second time joining the Red Sox this season, as he made five scoreless relief appearances from April 18-30. He had four strikeouts, no walks and five hits allowed in 6 1/3 innings of work. He is 3-2 with four saves and a 2.55 ERA this season for Pawtucket, racking up 56 strikeouts in 42 1/3 innings.