Stevens, 26, begins his third stint with the Cubs this season, having recorded a 5.71 ERA in 17 relief appearances from May 19 to June 26 and June 29 to July 5.
He held opponents to a .226 batting average in his first 15 big league outings before giving up six earned runs in his last two outings, July 2 and July 4 against Cincinnati.
With Triple-A Iowa this season, Stevens had a 3.18 ERA in 16 relief appearances. He gave up one home run in 22 2/3 innings.
Atkins, 24, had a 6.43 ERA in three relief outings for the Cubs since he was recalled July 7. With Iowa this season, Atkins was 5-2 with one save and a 3.60 ERA in 18 games (10 starts).
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.