The 21-year-old forward was the recipient of a hard hit along the end boards from Kings defenseman Matt Roy.
Nick Robertson will miss "significant time" with a shoulder injury as the injury bug has once again taken a bite out of the 21-year-old.
"It's a tough break, you feel for the kid," Maple Leafs head coach Sheldon Keefe said of Robertson. "The guy gives everything he has to the game."
The Toronto Maple Leafs forward left Thursday's game in the first period when he took a hard hit from Los Angeles Kings defenseman Matt Roy.
Robertson was slow to get up. He initially tried to before falling back onto the ice. After he was attended to by training staff, Robertson kept his right arm in a prone position.
The Leafs confirmed that Robertson sustained a shoulder injury and would not return to the game.
Roy was assessed a penalty for interference on the hit.
Auston Matthews initially filled in on the wing in Robertson's absence for the first few shifts after the players' absence.
Keefe lauded Robertson's stellar performance in the pre-season, but he started the season with the Toronto Marlies due to a lack of salary cap space on the roster and his ability to move up and down without requiring waivers.
There was a stretch where Robertson was a healthy scratch for four consecutive games. He did remerge and had played his third consecutive game in a top-six role after an injury to Calle Jarnkrok opened the door for his new opportunity alongside John Tavares and Mitch Marner.
Robertson's injury is another setback for the California native who has had a long history of injuries in a short period of time.
While with the Toronto Marlies last season, Robertson sustained a non-displaced fracture in his fibula and missed 10 weeks.
During the 2021 season, the forward suffered a knee injury in his regular-season NHL debut.
"He's had his fair share of injuries and it's a tough thing for him, but the good thing is he's bounced back and put in the work to continue to get better," Keefe said.
Robertson has two goals and five points in 15 games this season.