Mazzaro had a poor outing for Triple-A Omaha on Sunday, giving up five runs in 2 1/3 innings. He gave up just two hits, but had terrible command problems with seven walks, a hit batsman and a wild pitch. Among his 63 pitches to 17 batters, only 29 were strikes.
Given that information, Royals manager Ned Yost instead will start right-hander Sean O'Sullivan in Mazzaro's spot.
"We're going to leave him down there for another start or two," Yost said. "We'll set O'Sullivan in that spot when we get back home."
Mazzaro had won the Royals' No. 5 starter job in competition with O'Sullivan during Spring Training. But with no need for a fifth starter until Saturday because of open dates, Mazzaro was optioned to Omaha to prepare for that date.
"Sully can start in that spot, and we'll give Vinny a chance to get settled in and get a couple of good games under his belt and then see where we're at," Yost said.
Yost said that Mazzaro made two starts in extended spring camp at Surprise, Ariz., and made his first start for Omaha on Sunday at Albuquerque. Despite Mazzaro's problems, Omaha won, 9-6, with Mike Moustakas hitting his first home run.
O'Sullivan was put in the Royals' bullpen as the long man and has pitched in two games, giving up five runs in four innings for an 11.25 ERA.
"He's out of his role," Yost said. "He's a starter and we'll get him back in his role and see what happens."
The Royals have other options at Omaha in young pitchers such as Mike Montgomery and Danny Duffy, as well as veteran Jeff Suppan, but the plan is to get them more work as the Pacific Coast League season gets under way. Each of them has made just one start so far.
Mazzaro was obtained from the A's in last winter's trade for outfielder David DeJesus after going 6-8 with a 4.27 ERA for Oakland last year. O'Sullivan, acquired from the Los Angeles Angels in last summer's deal for third baseman Alberto Callaspo, was 3-6 with a 6.11 ERA after joining the Royals.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.