Overton's contract was selected by the Reds on Saturday ahead of his scheduled start against the Rockies.
Overton's first major-league start of the year comes in the tough environment of Coors Field. Through 19 innings for Triple-A Louisville this season, the 28-year-old owns a 2.84 ERA, striking out 22 batters while walking just two. Aristides Aquino was designated for assignment to clear space on the roster.