Robertson started his professional baseball career the following season, in 1996. He spent most of the season with the Gulf Coast League Astros, the rookie-class Astros franchise, where he played alongside future teammate Wade Miller. He also played one game with the Kissimmee Cobras. In 13 pitching appearances with the Astros, all of them starts, Robertson had a record of 5–3, a 1.72 ERA, and 98 strikeouts. For the 1997 season, Robertson was promoted to the Quad Cities River Bandits of the Midwest League. In 26 games for the River Bandits, Robertson had a record of 10–7, 135 strikeouts, and an ERA of 4.07.