Davis hadn't pitched since June 30 because of a strained right forearm. The right-hander was 1-0 with a 1.23 ERA and 19 saves in 30 games when he went on the DL, and the All-Star break helped limit the number of games he missed.
"One of the best relievers in the game," manager Ned Yost said. "It's hard to go a couple weeks without that, but we felt like we had it covered with [Kelvin] Herrera and [Luke Hochevar] and [Joakim] Soria, so that just gives us another quality arm we can put in the back end of that pen."
The Royals entered Saturday night's game at Detroit with a 34-5 record when leading after six innings, but one of the defeats came the previous night, when the Tigers scored three runs in the seventh off Hochevar to beat Kansas City 4-2.
Davis is one of the key components in a bullpen that has led the Royals to back-to-back American League pennants and a World Series title last season. He went 8-1 with a 0.94 ERA and 17 saves in 2015. The previous year, he was 9-2 with a 1.00 ERA.
Kansas City also optioned right-hander Brooks Pounders to Triple-A Omaha on Saturday.