Mills (back) was reinstated from the 60-day injured list Tuesday, Meghan Montemurro of the Chicago Tribune reports.
Mills recently made four rehab starts and posted a 4.97 ERA and 1.58 WHIP in 12.2 innings. He's served mainly as a starter for the Cubs over the last two seasons but will pitch out of the bullpen after being activated from the IL. However, the right-hander will be a top candidate to serve as a spot starter or enter the rotation at some point if needed.