Mainly a starting pitcher in the Royals minor league organization, he spent 6 years at the Single-A and Double-A levels before finally reaching Triple-A Omaha in 2002. During his minor league career, Calero excelled for the Royals organization. In 1996, he finished fourth in the Single-A Northwest League in ERA. He was also named to the Texas League All Star team in 1997. Through 2000, Calero had 31 wins in a Wrangler uniform and worked as a reliever in 2001 after 105 starts. The Royals traded Calero to the Philadelphia Phillies for shortstop Brian Harris in 2002 but was returned to the Royals and granted free agency.