The forward has missed three games, including a 5-4 win against the Columbus Blue Jackets on Tuesday, after taking a hit from teammate Jake Muzzin in practice on Friday.
"It was a little bit worse than the medical staff thought it would be," Maple Leafs coach Sheldon Keefe said Tuesday. "He had a chance to get an MRI, so they think it's going to be about 3-4 weeks. We'll stay on top of it, I think he'll be back on the ice sooner rather than later."
Marner was fourth on the Maple Leafs with 21 points (six goals, 15 assists) and averaged 20:47 of ice time in 24 games.
"He plays I don't know how many minutes but he's on the ice the whole game," Maple Leafs forward Alex Kerfoot said. "Everyone is going to have to step up. Not one guy is going to fill that role. We have to do it as a group. It has a trickle-down effect in our lineup, so we have to find a way to get results while he is out."
Defenseman Rasmus Sandin and Travis Dermott are each week to week. Sandin was injured when he was kneed by Winnipeg Jets defenseman Neal Pionk in a 6-3 loss on Sunday; he has eight assists in 25 games. Dermott has a shoulder injury and did not play Sunday; he has scored two points (one goal, one assist) in 18 games.
"We got good news on Rasmus, better than we were expecting given he left the rink on crutches unable to put any weight on his leg," Keefe said. "His MRI came back with no structural damage to his knee. It's just going to take some time to settle down, the swelling and bruising. They think once that settles he'll recover. It's not going to be long term, week to week, 2-3 weeks."
Forward Jason Spezza is unavailable awaiting an appeal of a six-game suspension he received from the NHL Department on Player Safety for kneeing Pionk on Sunday. He did not play Tuesday.
The Maple Leafs recalled defensemen Alex Biega, Kristians Rubins and forward Alex Steeves from Toronto of the American Hockey League.
Toronto hosts the Tampa Bay Lightning on Thursday (7 p.m. ET; TSN4, BSSUN, NHL LIVE).
NHL.com independent correspondent Dave McCarthy contributed to this report
