Gathright had a tight hamstring and was replaced by Alberto Callaspo with one out in the ninth as the Royals were about to wrap up an 11-3 victory over the Brewers.
"Just tightened up. That's why I went ahead and ran A.C. out there. I didn't want him to pull," manager Trey Hillman said.
"I was glad when he came in [because] I asked him immediately if he pulled and he said, 'No, just tight.' We'll see how he is tomorrow."
Gathright entered the game in the sixth inning and had an RBI single in his only at-bat. He's batting .333 (13-for-39) with eight stolen bases.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.