O'Nora, who suffered a mild concussion, was held overnight for observation. No stitches were necessary for his lacerated forehead, which was a bloody mess after he was struck.
Kansas City designated hitter Miguel Olivo's maple bat snapped as he hit a ground ball and the cupped end of the barrel whirled backward, striking O'Nora, who left the game after being treated by Royals trainers.
The game against the Rockies was completed with three umpires. For Wednesday night's series finale, Mike Muchlinski was called in to replace O'Nora.
Major League Baseball is currently discussing the safety concerns of the frequent shattering of maple bats. Olivo switched to an ash bat for his subsequent plate appearances.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.