"Per NHL protocol, he is unable to do the job we hired him for," general manager Jarmo Kekalainen told ESPN.
The outlet said that in August the NHL issued a memo to teams noting that individuals whose work involves being within 12 feet of club hockey operations personnel, including the players, must be fully vaccinated.
"Any person whose job, role, position or access entails or entitles them to have personal interactions (within 12 feet) with Club Hockey Operations personnel (including Players) are required to be Fully Vaccinated," a memo to teams stated, according to Elliotte Friedman.
Lefebvre had just been hired in June, according to ESPN: "Lefebvre was hired in June to join new head coach Brad Larsen's staff after serving as an assistant coach with the AHL San Diego Gulls, the Anaheim Ducks' affiliate, for three seasons," the outlet reported.
Cleveland Monsters Associate Coach Steve McCarthy will now fill the role instead.
"While we are disappointed, we respect that this decision is a personal one for Sylvain and wish him well," Kekalainen said. "We feel fortunate to have an outstanding coach join our club in Steve McCarthy, who has played in this league, won championships as a player and been an important member of our hockey operations department as an assistant coach in Cleveland."
Unvaccinated hockey players will face restrictions during the season, particularly when traveling, according to ESPN, which also noted that the NHL is giving athletes the opportunity to opt out of the season because of personal health concerns prior to an Oct. 1 deadline.