Mills (back) was transferred to the 60-day injured list Thursday, Tim Stebbins of NBC Sports Chicago reports.
Mills was cleared to face hitters this weekend and threw a bullpen session Thursday, but he'll now be moved to the 60-day injured list to make room on the 40-man roster. The right-hander will likely require several rehab games before rejoining the Cubs, so Thursday's transaction is unlikely to alter his recovery timetable.