With one out and Mark Teahen at the plate, Eaton took off from second base.
"It was one of those things where he didn't think I was paying attention," Shields said. "Moustakas was right on top of it, he yelled, 'Step off,' and we made the play."
Shields threw to Moustakas, who tagged Eaton as they banged into each other. Moustakas went to the dugout to get his cut lip treated by trainer Nick Kenney but stayed in the game.
"Just a guy not knowing the game, that's all it is," Moustakas said later. "Just doesn't have a feel for the game, that's it. That's all I've got to say."
For a moment, it looked as if sparks might fly before Eaton trotted off the field.
"He tried to catch everybody with their head down but, of course, we didn't have our head down," manager Ned Yost said. "Shields threw it to Moose and [Eaton] just kind of forearmed him in the lip and cut his lip a little bit."
"He got a little bloody out there, but that's baseball," Shields said.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.