The Cubs reinstated Thompson (rib) from the 15-day injured list and optioned him to Triple-A Iowa, Meghan Montemurro of the Chicago Tribune reports.
Thompson had been on the shelf since late June with a right rib fracture but is back to full health after making four rehab appearances. The Cubs don't have a spot in the bullpen available for Thompson at the moment, but the right-hander should rank near the top of the list for a call-up if the club needs a fresh relief arm at any point.