Chen was supposed to start Saturday against Detroit, but he struggled with the pain in a side session on Thursday, even after receiving an epidural that was supposed to alleviate it.
Afterward, Royals manager Ned Yost acknowledged that a DL stint was likely.
Left-hander Danny Duffy will slide from the bullpen into Chen's spot in the rotation.
Brooks, who was 0-1 with a 4.80 ERA at Omaha, provides some bullpen depth. He spent five days with the Royals in early April but did not appear in a game.
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