It’s not clear which slur was used, nor who might have uttered it, nor where.
But the league is investigating nonetheless.
It won’t offer any further comment until its probe has been finished.
In a statement, Toronto Maple Leafs general manager Kyle Dubas said the club is “cooperating fully” with the NHL in this matter.
“The issue of homophobia is one the Toronto Maple Leaf Hockey Club strongly condemns and takes very seriously,” he said.
The slur allegedly happened in a game that the Leafs lost to the Lightning, 6-2.
Cedric Paquette and Tyler Johnson each scored two goals for the Lightning, while the remaining ones were marked by Anthony Cirelli and Ondrej Palat.
WATCH: Pacquette, Johnson power Lightning to win over Maple Leafs
Auston Matthews and Connor Brown scored for Toronto.
This isn’t the first time that the issue of homophobia has cropped up in connection with NHL hockey.
Anaheim Ducks captain Ryan Getzlaf was fined $10,000 after he uttered a remark in a 2017 playoff game against the Nashville Predators.
“There was obviously some words said, not necessarily directed at anyone in particular,” Getzlaf said at the time.
“I’ve got to be a little bit more responsible for the words I choose. It’s tough to see someone refer to it as [homophobic]. I didn’t mean it in that manner in any way.”
And in 2011, then-Philadelphia Flyers player Wayne Simmonds faced scrutiny for allegedly directing a homophobic remark toward then-New York Rangers player Sean Avery.
The NHL did not suspend Simmonds at the time because it was “unable to substantiate with the necessary degree of certainty what was said and by whom,” said a statement at the time.