Mermis was injured in Monday’s contest against the Tampa Bay Lightning after taking a knee-on-knee hit from forward Gage Goncalves, which led to a scrum and multiple penalties.
Mermis had appeared in 11 games with the Maple Leafs on the season, scoring once.
Thrun, 24, was acquired from the San Jose Sharks during the off-season in a deal that sent Ryan Reaves the other way.
Although he’s yet to make his Maple Leafs debut, Thrun skated in 119 career NHL games with the Sharks, tallying five goals and 20 assists.
Meanwhile, another defenceman was set to miss practice Wednesday as Morgan Rielly was absent due to illness.
In better news for the Maple Leafs, injured goalie Joseph Woll, who has not played since sustaining a lower-body ailment on Dec. 4, returned to the ice. He is eligible to come off the IR on Thursday.
