Jeffress joined the team before Monday night's game against the Cincinnati Reds. He was 4-1 with a 1.51 ERA and five saves in the minors.
The 26-year-old Jeffress has returned to a team that drafted him in the first round in 2006. He was traded to Kansas City in the deal that brought Zach Greinke to Milwaukee in 2010.
Jeffress has been suspended twice for violating the minor league drug policy, which he has said was for marijuana use. He has also suffered from debilitating seizures.
Jeffress said Monday he was clean and sober, and that he has his seizures under control.
Reliever Rob Wooten was optioned to Triple-A to make room for Jeffress.