Adrian Dater, who covers the Avalanche for Colorado Hockey Now.com, reports that the injury is a broken collarbone.
Rantanen was injured in the second period after being tripped by the Lightning’s Erik Cernak and sliding hard into the boards behind the Lightning net. The 23-year-old was slow to get up after the collision and immediately left for the Avalanche dressing room. Cernak was given a tripping penalty on the play and Rantanen did not return to the game.
Rantanen has 19 goals and 40 points in 41 games this season. He missed more than a month earlier in the season with a lower-body injury.
Rantanen joins a growing list of injured Avalanche players as the team looks to hold on to a playoff spot in the Central Division. Other players out long term with various ailments include forwards Nazem Kadri and Matt Calvert, and goaltender Philipp Grubauer.
With less than a week to go before the NHL trade deadline, all eyes will be on the Avalanche to see whether the club decides to add to their roster in the wake of these injuries. After play Monday — in which the club lost 4-3 in overtime to the Lightning — the Avalanche sit third in the Central with 73 points, one point behind both Dallas and St. Louis and eight points ahead of Nashville and Winnipeg.