With a 4.1 seconds left in the first period, Killorn boarded Ranger, who went down to the ice and remained there for almost 10 minutes. The defenceman moved his legs but not much else after the hit.
A stretcher was brought onto the ice as Ranger's Leafs teammates huddled around and Tampa Bay players Ryan Malone, Mike Kostka, Teddy Purcell and Ryan Malone remained on the ice and watched from afar well after officials decided to end the period early while medical attention was being administered.
Killorn was given a five-minute major penalty for boarding and a game misconduct.
Replays appeared to show Ranger turn his head before the hit to see Killorn coming. When the Lightning forward finished his check, Ranger's head struck the glass hard.
The hit immediately caused a scrum in the corner to goaltender James Reimer's right. That quickly gave way to trainers helping Ranger.
On their official Twitter account, the Leafs wrote: "We'll update you on Paul Ranger's condition when we know more."
Ranger, who was dressed for the 48th game this season, has four goals and eight assists. The 29-year-old made his return to the NHL this season after leaving his last team, the Lightning, abruptly early in the 2009-10 season.