The 29-year-old could have elected to become a free agent.
Mills was designated for assignment by Toronto on Monday to open a roster spot for Adam Lind, who was activated from the 15-day disabled list. In his time with Toronto, Mills had an ERA of 27.00 in 4.1 innings.
In 20.2 innings this season, Mills has an ERA of 9.15 and 19 strikeouts.
- Sarah Barshop