"Last night, he started experiencing some stomach problems, some gas pains," manager Ned Yost said before Saturday night's game against the Tigers. "They gave him some fluid and they thought it might have been food poisoning. He woke up again this morning and he had to have an appendectomy at 10 o'clock."
Pena is expected to be released from Henry Ford Hospital on Sunday morning, enabling him to fly back to Kansas City with the Royals.
"He did great," Yost said. "Everything went well."
Pena, who was released by the Astros on July 31 after hitting .209, signed a Minor League contract with the Royals on Aug. 27 and was assigned to Triple-A Omaha. He played in five games for the Storm Chasers before joining the September callups to Kansas City.
Pena played in four games and struck out in all three of his at-bats. Now, he's in a hospital.
"The recovery time is about two weeks," Yost said. "That's also the time left in the season."
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.