The 22-year-old was hurt midway through the third period of Wednesday’s 6-2 victory over the Oilers after getting checked into the boards by Edmonton forward Brad Malone.
Dermott was hunched over in pain favouring his left shoulder as skated to the locker room accompanied by a trainer.
In his first full NHL season, Dermott has four goals and 13 assists in 60 games with Toronto in 2018-19. Selected 34th overall at the 2015 draft, the native of Newmarket, Ont., has 30 points (five goals, 25 assists) in 97 career outings.
The injury comes at an especially bad time for the Leafs, who were already minus defenceman Jake Gardiner because of a sore back. The 28-year-old, who has two goals and 27 assists in 60 games this season, is listed as week to week.
Head coach Mike Babcock said following Wednesday’s victory that the team would recall blue-liner Martin Marincin from the Toronto Marlies of the American Hockey League in Dermott’s place.