Chen will be placed on the 15-day DL on Wednesday, retroactive to May 6, with a left lat strain. He could miss only two starts if he could return by May 21, the first date he is eligible to come off the DL.
Chen (4-1) leads the club in victories and has a 3.59 ERA in seven starts. His dozen victories last year topped the team. He re-signed with the Royals in January after declaring for free agency.
Chen was scheduled to start Wednesday at New York, but instead Mazzaro will face the Yankees.
Mazzaro went 1-0 with a 4.97 ERA in five starts with Omaha. In 25 1/3 innings, he had allowed 15 runs, 14 earned, and given up 31 hits, while striking out 27 and walking 13.
Mazzaro, acquired in a Nov. 10 trade with Oakland for outfielder David DeJesus, was 6-8 with a 4.27 in 24 games, including 18 starts, last year with the Athletics.
He will be the sixth starter used by the Royals this season.
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