Rivera was optioned to Triple-A Omaha on Thursday.
Rivera lost out on playing time recently after Adalberto Mondesi returned from the injured list, and he'll likely see more consistent at-bats in the minors. The 25-year-old has hit .256 in 29 games during his first major-league season and will be an option to rejoin the Royals if they need depth in the corner infield positions down the stretch.