The diminutive forward was spilled while going into a board battle with Kings defenceman Matt Roy, falling awkwardly into the boards and remaining down.
Robertson would eventually require help off the ice by trainers, while the Maple Leafs received a power play for interference on the mishap.
The 21-year-old had two goals and three assists in 14 games heading into Thursday’s contest, as he’s bounced in and out of the Maple Leafs lineup.
When speaking to the media post-game, head coach Sheldon Keefe announced Robertson’s injury will result in him missing significant time.
The Maple Leafs are already down centre Calle Jarnkrok (groin), and defencemen Victor Mete (lower-body), Jake Muzzin (neck), Morgan Rielly (knee) and Jordie Benn (upper-body).
Toronto did receive a boost to its blue line Thursday when T.J. Brodie returned to the lineup after a 12-game absence.
The Maple Leafs beat the Kings 5-0.