A club spokesman said that Guillen was running a 101-degree fever and had flu-like symptoms. The Royals' clubhouse has been hit hard by the flu bug during camp.
The Royals gave Guillen the option of getting a late start playing Cactus League games. He has been in just three of the 15 games so far, with an RBI single in eight at-bats.
Guillen went 0-for-3 in Wednesday's game against the Angels at Tempe, Ariz.
He's in the first year of a three-year, $36 million contract he signed with the Royals in December. Pending against him is a 15-day penalty at the start of the season for violating Major League Baseball's Drug Prevention Program.
The Associated Press reported on Wednesday that no hearing dates have been scheduled to hear the grievance filed by the Players' Association on behalf of Guillen.
Sen. George Mitchell recommended that players named in his report not be penalized. The penalty could be rescinded by the Commissioner or put on hold pending a decision by an arbitrator, according to the AP.
Commissioner Bud Selig said on Saturday that no decision had been made in the Guillen case.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.