Fleury immediately made his NHL debut in 2003–04 as the youngest goaltender in the league at 18 years old (three years younger than the second-youngest, Rick DiPietro of the New York Islanders). He appeared in his first NHL game on October 10, 2003, against the Los Angeles Kings, recording an impressive 46-save performance, which included a penalty shot save, in a 3–0 loss. Fleury recorded his first NHL win in his very next start, on October 18, with 31 saves in a 4–3 win over the Detroit Red Wings. His first NHL shutout came on October 30, in a 1–0 victory over the Chicago Blackhawks. Fleury shared time with goaltenders Jean-Sébastien Aubin and Sébastien Caron and lived up to first-overall-pick expectations early, earning Rookie of the Month honours in October with a 2–2–2 record, 1.96 goals against average (GAA) and .943 save percentage. As the season progressed, however, his performance began to sink, mainly due to Pittsburgh's poor defence. The team regularly gave up over 30 shots per game, and rarely managed to become an offensive threat. He was loaned to Team Canada for the 2004 World Junior Championships in December and, upon returning with a second consecutive silver medal, he was sent back to the QMJHL on January 29, 2004. In light of financial difficulties for the franchise, it is believed Fleury's $3 million contract bonus, which he would have potentially received if he stayed and met several performance goals, was a factor in the decision to return him to Cape Breton. To no avail, Fleury offered to forfeit his bonus to remain with the club. Fleury finished the QMJHL season with Cape Breton in a first round elimination and was subsequently assigned to Pittsburgh's American Hockey League (AHL) affiliate, the Wilkes-Barre/Scranton Penguins, and appeared in two post-season games.