The Blue Jays made the roster move before opening a key three-game series in Seattle. Second baseman Ryan Goins was optioned to Buffalo after Toronto's 19-inning victory over Detroit on Sunday.
Mills' role with Toronto will be as protection for the bullpen after seven relievers were used in the win over Detroit. Mills was 1-0 with a 3.14 ERA in three starts with Buffalo after being claimed off waivers last month.
Goins appeared in 43 games for Toronto this season. He is hitting .197.