For the past two days, Dempster has been dealing with a stomach virus that eventually locked up his back. He spent Saturday night in a Chicago hospital.
"I stayed in the hospital last night for some tests," Dempster said. "They wanted to make sure everything was all right. As the days have gone by, I eventually got pretty locked up in my back. But I feel much better today."
Dempster said he was placed on an IV on Saturday night, undergoing multiple tests, including one for kidney stones, which were ruled out.
"They feel it was something gastrointestinal," Dempster said. "This will push my start back a few days, or I'll miss one and make my next one."
If Dempster is pushed back to his next start, he would be scheduled to pitch Saturday in Pittsburgh. It would be his last one before the All-Star break.
Dempster said the illness was unrelated to the sore hip he suffered in Cincinnati in mid-May. In that outing against the Reds, he was almost scratched while warming up in the bullpen but came out and pitched six innings.
Dempster is the fifth Cubs starter from the Opening Day rotation to miss a scheduled start. The other four Cubs pitchers, Andrew Cashner, Randy Wells, Matt Garza and Carlos Zambrano, have all spent time on the disabled list. Zambrano was put on the DL on Thursday with lower back soreness.
Bruce Levine covers the Cubs for ESPNChicago.com and ESPN 1000.