On June 25, , Byrnes was placed on the disabled list with broken left hand. He was injured during a game against the Texas Rangers after opposing pitcher Scott Feldman hit Byrnes on the hand with a pitch. After he sustained the injury, he was replaced by Ryan Roberts. He was reinstated from the disabled list on September 5, 2009. He played 16 minor league games during his rehab stint with the Triple-A Reno Aces, batting .279 with seven doubles, one triple, two home runs, nine RBIs, and one stolen base. On January 15, 2010, Byrnes was designated for assignment by the Arizona Diamondbacks to make room on the 40-man roster for Adam LaRoche.