Gary Thorne of MASN reported in the broadcast of the August 27, 2013 game between the Orioles and the Boston Red Sox that Castro succeeded Rick Adair because Adair had taken a leave of absence for personal reasons starting in August 2013.

Late in the 2002 season, Castro also briefly served as pitching coach after the resignation of Dave Stewart.

He then returned to his bullpen role until he was named pitching coach by new Milwaukee manager Ken Macha on November 7, 2008.

Castro was subsequently fired as pitching coach on August 12, 2009, and replaced with former Brewer Chris Bosio.

Castro was the pitching coach for the Dominican Republic's entry in the inaugural World Baseball Classic.