The 19-year-old, who made his NHL debut Aug. 2 in the Stanley Cup Qualifiers, was quarantining in Toronto while the United States held its selection camp in Plymouth, Michigan. The Maple Leafs did not make him available for the United States, which announced its 25-player roster on Saturday.
The WJC is scheduled to be played at Rogers Place in Edmonton from Dec. 25 to Jan. 5, 2021.
"Based on the NHL and NHLPA health and safety protocols, unknowns surrounding roster protocols given the coronavirus, and Nick's standing on our roster heading into training camp, we feel it is in the best interest of both Nick and the Maple Leafs that he remain in Toronto so that we ensure he can participate in our training camp, while abiding by all health and safety standards from the City of Toronto and Province of Ontario," Maple Leafs general manager Kyle Dubas said Saturday.
Based on the IIHF rules for the tournament, Robertson would have to join the United States in Edmonton no later than Sunday. All players and staff members for each of the 10 teams must quarantine upon arrival for five days in their hotel rooms and be tested daily, and no one is able to arrive afterward.
A start date for NHL training camps and the regular-season schedule has not been determined. NHL protocols call for a seven-day quarantine for a player who arrives in his NHL city on a commercial flight. If Robertson were in Edmonton for the tournament and returned to Toronto, he would need to quarantine for seven days and could miss most of training camp.
"This was a hockey decision by the Toronto Maple Leafs, and I think we can all respect hockey decisions," U.S. general manager John Vanbiesbrouck said Saturday. "They wanted Nick to be ready for the start of when their camp starts, which is what we were told. We went down this road to have him as a strong possibility all the way along and then it looks like there's some hard dates coming back."
Robertson, selected by the Maple Leafs in the second round (No. 53) of the 2019 NHL Draft, scored in a 4-3 overtime loss to the Columbus Blue Jackets in Game 3 of the Qualifiers, becoming the first 18-year-old with a postseason goal for the Maple Leafs since Ted Kennedy on March 28, 1944.
"Getting thrown in with good players during camp definitely helped my comfort level," Robertson said during the postseason. "Getting to know the guys off the ice too definitely helps on the ice. I'm learning a lot from the guys and am asking them a lot of questions which definitely helps me. The expectations are probably high because everything is a lot better than what I was exposed to at the junior level."
Robertson led the Ontario Hockey League with 55 goals in 46 games for Peterborough last season. The California native scored five points (two goals, three assists) in five games for the United States at the 2020 WJC.
"The big thing we've seen with him throughout training camp is the way he's been able to work, his work habits and the way he skates and the way he's been on top of the puck," Maple Leafs coach Sheldon Keefe said during the postseason. "What we've said all along with him is that he's a guy with a great skill set who can that if he gets an opportunity, he can make a difference offensively."
The 2021 WJC is scheduled to be played with no fans in attendance, and the teams will be in a secure zone similar to the one used by the NHL during the Stanley Cup Playoffs. Games will be broadcast in the United States on NHL Network.
NHL.com staff writer Mike Zeisberger contributed to this report