After being demoted back to Buffalo, the Indians traded Robertson on August 3 to the Montreal Expos for Pierre-Luc Marceau. He pitched in seven games for the Edmonton Trappers to end the season, finishing with a 1–3 record and a 5.73 ERA. He was on the Expos' 40-man roster for September, but did not play a game for them, and in October he was removed from the roster. In November, the Cincinnati Reds signed him to a minor league contract with an invitation to spring training. During spring training in 2005, Robertson competed with Aaron Harang, Brandon Claussen, Luke Hudson, and Josh Hancock for two spots in the Reds' rotation. At the end of spring training, he was assigned to the Reds' minor league squad, the Louisville Bats. Robertson spent the season there, pitching in 28 games, 18 of them starts, and ended the year with a 5–11 record and a 5.46 ERA. After the season ended, Robertson was released, becoming a free agent.