Humber spent most of the 2008 season assigned to the Twins' Class-AAA affiliate, the Rochester Red Wings of the International League. He started off slowly with a 0–5 win–loss record and a 5.83 ERA in his first nine starts. With Rochester, Humber eventually compiled a 10–8 win–loss record with a 4.56 ERA in 31 games. He finished with a 6–1 win–loss record and a 2.67 ERA in the second half. His 4–1 win–loss record with a 2.74 ERA in August earned him the Minor League Pitcher of the Month Award. The Twins promoted Humber in August 2008, and he appeared in five games for the Twins, all in relief. Humber pitched innings, and posted a 4.63 ERA and six strikeouts for the Twins. Following the 2008 season, Humber was out of options, and could not be sent to the minor leagues without first being subjected to waivers, where other teams could claim him.