Plaza shifted to second base with the Rochester Red Wings in 1956, and batted .297 with five home runs and 30 RBI in 121 games during his first season in Triple-A.[3] His batting average slipped to .221 his second season with Rochester, however, he hit a career-high 14 home runs and had 49 RBI in 144 games.[3]
Plaza's first major league coaching job was the first base and hitting coach for the Seattle Pilots in 1969. Pilots General Manager Marvin Milkes let Plaza go along with the rest of the coaching staff as the team struggled with bankruptcy and a host of other issues after completing their one and only season in Major League Baseball.[6] His term with the Pilots earned him mention in Jim Bouton's book Ball Four, as "The Drill Instructor."
Plaza coached in the Cincinnati Reds' farm system following his stint in Seattle, and joined the big league club following the 1977 season.[7] After succeeding Alex Grammas as Cincinnati's third base coach in 1979, he was shifted to first base coach in June of 1979 by manager John McNamara because of Reds' baserunners being thrown out at home plate after being waved in by Plaza.[8]