As the 2003 season began, Robertson was competing alongside Brad Lidge, Kirk Saarloos, and Pete Munro for the final spot in the Astros' starting rotation. He pitched the entire month of spring training, and when the season began he was granted the final spot in the rotation. Robertson made his season debut on April 3, and allowed eight earned runs in innings, losing the match against the Colorado Rockies 10–5. He got his first career victory on April 10, allowing two hits in seven innings in a 4–2 victory against the Cincinnati Reds. He struggled in his next three starts, and by the end of April Robertson had a 1–3 record and an ERA of 7.99, which caused manager Jimy Williams to consider moving him back to the bullpen. After a May 2 matchup against the Atlanta Braves further increased his ERA, Robertson was demoted to the minor leagues, but only played with the Zephyrs for one game before being placed back on the major league roster ten days later.
