The Reds also announced that they had signed utility player Willie Harris, a free agent from the Mets.
Valdez, 33, has played six seasons in the majors with the White Sox, Mariners, Padres, Dodgers, Mets and Phillies. He batted .249 last year with one homer and 30 RBIs in 99 games. The second baseman also notched the victory in a 5-4, 19-inning win over the Reds in Philadelphia on May 25, holding the Reds hitless in the final inning.
Horst, 26, reached the majors for the first time last season, making a dozen relief appearances with a 2.93 ERA.