"He's sore," manager Trey Hillman said. "I'm nervous about putting him in the outfield. I'm going to give him a day of rest today."
Guillen said he re-injured the groin while running to first base on an RBI single in the fifth inning of Saturday's game. Hoping he could help the Royals by delivering a big hit late in a tight game, Guillen stayed in the lineup as the right fielder. But playing hurt backfired when he was unable to reach a pop fly in the eighth inning, leading to a game-winning Tampa Bay rally.
Hillman said he hopes to get Guillen back in the mix this week in Oakland, where the A's throw left-handers Dallas Braden and Greg Smith against the Royals on Monday and Tuesday.
Guillen had expressed concern late Saturday about playing in the homestand finale.
"It's not a good one to go [out] there and try to be a superhero," Guillen said.
Weighing the pros and cons of playing with a mobility-limiting injury is difficult, according to Hillman.
"When something negative like that happens and it affects the club, [Guillen] doesn't like that," Hillman said. "And he doesn't like not playing."
Using Guillen as a designated hitter isn't the answer because the groin could give way as Guillen breaks from the box or runs the bases. Hillman said Guillen is a pinch-hitting possibility for Sunday.
"Jose didn't know there was going to be a ball that was going to fall in front of him," Hillman said, referring to B.J. Upton's crucial bloop double on Saturday. "His mindset is staying in for maybe having the chance to help the club."
Robert Falkoff is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.