The extent of the injury wasn't immediately known and Moustakas was scheduled to have an MRI.
"He's got some soreness in his knee and we're just trying to determine what it is. There's no swelling, which is good, and he's got pretty good strength," Royals manager Ned Yost said.
Moustakas made a spectacular stab of Casper Wells' hot smash down the line, lunging to spear the ball in foul ground and throwing from his knees to first baseman Eric Hosmer. Wells was called safe on a close play. Afterward, Moustakas felt discomfort in the knee.
"The ball's down the line, I got it, my leg is underneath me and I threw from my knees. When I threw I must have twisted my body around a little too much and just felt a little something in my knee," Moustakas said. "I put a little pressure on it and was walking around, and it felt OK, but Ned wanted to get me out of there."
Yuniesky Betancourt moved from second base to third and Chris Getz took over at second base in the bottom of the second inning.
After the game, Moustakas was feeling OK.
"It feels real good and I got the rest of the day off, and the skip said he was going to give me [Sunday off], but I'll be ready to go," he said.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.