Guillen was listed as the cleanup hitter and left fielder in manager Trey Hillman's original lineup for the first game after the All-Star break against the Chicago White Sox. Instead, designated hitter Billy Butler batted fourth and Esteban German was in left field.
Guillen vacationed in Las Vegas and hit in a batting cage for three days during the All-Star break. Hillman reported that Guillen was struck by the back spasms in the middle of the night after arriving in Chicago.
"The middle of his back was badly spasmed, he was knotted up and I really don't project him to be available today," Hillman said.
"Hopefully, we can get him ready for tomorrow. It's not really the way we wanted to start out -- down without the No. 4 guy, but it is what it is."
Guillen contacted head athletic trainer Nick Swartz on Thursday night and underwent treatment.
At the break, Guillen's 65 RBIs and 29 doubles each ranked fourth in the American League. He also led the Royals with 13 home runs and 99 hits. Previously, he missed just four games although he has been plagued by hip and groin ailments. A week ago, he sat out a game because of a stiff neck.
Hillman obviously would like to have a completely healthy Guillen.
"That would be nice, because we see the production level that can be there and, hopefully, he can get to a point where his body will allow him to go out there and give it what he's got every day," Hillman said.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.