Dermott joins Auston Matthews and Zach Hyman among Leafs requiring offseason surgery, while John Tavares recently left the Canadian World Championship squad after suffering an oblique injury.
The news on Tavares came shortly before the Maple Leafs said defenseman Travis Dermott will have shoulder surgery and miss at least six months.
The Leafs tweeted that Tavares will be evaluated by the club's medical staff with a further update in coming days.
"We are disappointed that John will not be able to participate as a member of Team Canada", Hockey Canada official Jason Botterill said in a statement.
It is likely that the surgery is to fix the damage from that injury and with a minimum six month recovery time, that will make Dermott unavailable for all of training camp and into the regular season.
The 28-year-old was set to represent Canada a fourth time at the IIHF World Championship after winning gold at the 2014 Olympic Winter Games.
Dermott had four goals and 13 assists in 64 games this season, plus a goal and two assists in seven playoff games.
Vegas Golden Knights winger Mark Stone and Philadelphia Flyers centre Sean Couturier will be counted on to pick up the slack for Team Canada, which opens play Friday against Finland at 10:15 a.m. ET.
In seven games in the first round against the Boston Bruins, Tavares had two goals and three assists. He was on Canada's championship team at the 2014 Olympics, where he injured his left knee and missed the rest of that National Hockey League season.