The 25-year-old Horst had a 1.73 ERA in 12 relief appearances for Louisville. He has never pitched in the major leagues.
Horst is the second Louisville pitcher to join the Reds in two days. Mike Leake was recalled and earned the win on Friday night.
Right-hander Homer Bailey was placed on the disabled list with a sprained right shoulder, retroactive to Friday. Right-hander Daryl Thompson, who gave up five runs in three innings in his only appearance with Cincinnati, was optioned to Double-A Carolina.