In 51 pitching appearances (32 starts), Robertson posted a win–loss record of 16–12 with a 5.71 earned run average and 111 strikeouts in 184+1⁄3 innings of work.

[3] The Houston Astros selected Robertson in the 24th round of the 1995 Major League Baseball draft, and he signed a contract with them on August 22, 1995.

[9] In 27 starts for the Zephyrs, Robertson had a 12–8 record, a league-leading 2.55 ERA, and 114 strikeouts en route to being named the Pacific Coast League Pitcher of the Year.

[4] 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.

[14] Robertson made his season debut on April 3, and allowed eight earned runs in 4+1⁄3 innings, losing the match against the Colorado Rockies 10–5.

[16] 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.

[18] In a May 22 matchup against the St. Louis Cardinals, Robertson allowed one run and four hits in a 5–2 victory; after the game, catcher Gregg Zaun said that it was "absolutely the best he's thrown.

[21] His streak ended on July 27, as he only lasted 1+2⁄3 innings in a 5–3 loss to the Chicago Cubs in what was at the time the shortest start of his major league career.

[22] A month later, Robertson broke the team record for most victories by a left-handed rookie pitcher when he notched his 13th win against the Los Angeles Dodgers on September 2.

As a result, Robertson found himself battling with Tim Redding, Jared Fernandez, Carlos Hernández and Brandon Duckworth for the final spot in the rotation.

[25] After Roger Clemens was signed as well, the Astros decided before spring training began that Robertson would pitch out of the bullpen for the season.

On March 31, just before the season started, the Astros traded Robertson to the Cleveland Indians for Luke Scott and Willy Taveras.

[5] The Indians called Robertson up to the major league roster on June 25 after placing Rafael Betancourt on the disabled list.

[4] In his final appearance in the major leagues, a July 21 game against the Chicago White Sox, Robertson was ejected after hitting Magglio Ordóñez with a pitch in the fifth inning.

[31] During spring training in 2005, Robertson competed with Aaron Harang, Brandon Claussen, Luke Hudson, and Josh Hancock for two spots in the Reds' rotation.

[5] After being released from the Mets, Robertson spent the rest of the season pitching for the Uni-President Lions of the Chinese Professional Baseball League.

[35] After a month there, in which he pitched four games, going 1–3 with a 5.54 ERA,[5] he returned to Newark to finish the season with the Bears, in what was his last professional baseball appearance.