Cain raced deep toward the wall to snag Daric Barton's drive while the A's Collin Cowgill, not anticipating a catch, had left first base and rounded second. Cowgill retreated, but Cain's throw was relayed by Chris Getz to Eric Hosmer at first and it turned into a spectacular yet easy double play.
When Cain's turn to bat came up in the fourth, Mitch Maier pinch-hit for him and then replaced him in center.
Manager Ned Yost had planned to start Maier in Cain's place on Wednesday, so the injury probably cinches that move.
"He pulled the groin when he hit the wall," said Yost. "It's just slight, it's a Grade 1. We took him out because it was wet out there. We planned on giving him the day off [on Wednesday] anyway, so we look to have him back in there Friday."
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.