Mijares is one of two left-handed relievers in the Twins bullpen. He's tied for the team lead with 18 appearances in 37 games this season, posting a 5.40 ERA with 11 walks in 11 2/3 innings.
Right-hander Jim Hoey was also optioned to Triple-A Rochester on Sunday as the Twins shuffled their struggling bullpen, which entered the game with the second-worst ERA in the league.
To fill in for Mijares, the Twins selected the contract of lefty Phil Dumatrait from Rochester where he posted a 1.15 ERA in 15 2/3 innings. To replace Hoey, right-hander Anthony Swarzak was recalled from Rochester. He had a 3.90 ERA in six starts.
To make room for Dumatrait on the 40-man roster, catcher Steve Holm was designated for assignment.