The 19-year-old forward left his NHL regular-season debut in the first period of a 3-2 win against the Ottawa Senators at Canadian Tire Centre on Jan. 16.
"He had an MRI Sunday," Maple Leafs coach Sheldon Keefe said Monday. "Results we got back were better; we would classify it as good news considering how it was looking but he is going to miss some time. We figure it will be at least four weeks but we took that as good news."
The Maple Leafs (2-1-0) play the Winnipeg Jets (1-0-0) at Scotiabank Arena on Monday (7 p.m. ET; TVAS, SNO, TSN3, NHL.TV).
Robertson, a second-round pick (No. 53) in the 2019 NHL Draft, scored in his NHL debut, a 4-3 overtime loss to the Columbus Blue Jackets in Game 3 of the Stanley Cup Qualifiers on Aug. 2. He was the first 18-year-old with a postseason goal for the Maple Leafs since Ted Kennedy on March 28, 1944.
NHL.com Independent Correspondent Dave McCarthy contributed to this report
