Sullivan was removed from his start Friday against the Athletics due to an illness, Patrick Saunders of The Denver Post reports.
Sullivan gave up two hits and zero walks while striking out two batters in three shutout innings, but his MLB debut ended prematurely once he began to feel under the weather. Despite his brief outing, the 23-year-old southpaw may have pitched well enough in an extremely hitter-friendly environment to earn another start with the big club.