Mills was designated for assignment earlier Wednesday after the Royals signed free agent pitcher Jason Hammel, who played last season for the World Series champion Cubs.
The 25-year-old Mills made his major league debut last year, going 0-0 in three games and giving up five runs in 3 1/3 innings. He was a combined 5-5 with a 3.22 ERA at Triple-A and Double-A.
The 23-year-old Dewees hit a combined .284 with 74 RBIs and 31 steals at two Class A levels.
Lefty David Rollins was designated for assignment by the Cubs to make room for Mills on the 40-man roster. It’s been a busy offseason for the 27-year-old Rollins, who pitched 11 games for Seattle last season. Since then, he’s been claimed off waivers twice by the Cubs, twice by Texas and once by Philadelphia.