The Cubs placed Thompson on the 15-day injured list Friday with a right rib fracture, retroactive to Thursday, Taylor McGregor of Marquee Sports Network reports.
Thompson has fired 5.1 scoreless innings since returning from Triple-A Iowa on June 16, but he'll now be sidelined for at least the next two weeks as a result of his injury. Ethan Roberts will come up from the minors to fill the vacancy in Chicago's bullpen.