Robertson pitched in 14 games for the 2004 Buffalo Bisons squad that eventually won an International League Governors' Cup championship. Acquired by the Cleveland Indians in a spring-training trade with the Houston Astros, the southpaw won four games in 12 starts and a pair of relief appearances for the Herd. Robertson received two promotions to Cleveland that year before being traded again to the Montreal Expos in August.

Robertson still holds the Astros' win record by a rookie after he went 15-9 in 2003 in his first big league season. He was also signed by the Mets on May 10, 2006 and pitched in 11 games with New York's then-Triple-A affiliate in Norfolk.