Mazzaro was 4-3 with a 5.73 ERA for the Royals while filling in as a spot starter because of injuries last season. Robinson struck out twice in his first four major league at-bats. Both were designated for assignment on Nov. 20.
The 21-year-old Santos pitched for both Dominican Pirates clubs last season, combining for a 6-3 record and a 2.44 ERA in 14 games, including 12 starts. In 62 2-3 innings, he allowed 42 hits, walked 20 and struck out 74 while holding opponents to a .184 average.
Rico, who turns 19 on Thursday, was 0-1 with a 7.04 ERA for the Dominican Pirates.
Volstad and Pena also were designated for assignment on Nov. 20.