Robertson made his MLB debut on September 2, 2002, against the Texas Rangers. He was starting in place of Roy Oswalt, who was serving a suspension for hitting a batter with a pitch. He allowed two runs in innings and got the loss in his only start that year. He got his second decision in a 5–2 loss to the San Francisco Giants on September 28. In the game, the Giants clinched a playoff spot, and Barry Bonds hit a home run off Robertson into McCovey Cove to seal the game for them. He finished the season with a 6.52 ERA in innings pitched.