Rantanen was skating on the far side of the ice early in the second period when he appeared to slip and fall. He went down in pain briefly before somehow managing to skate off the ice without officials stopping the game.
Video from the game showed Rantanen's left ankle twist backward before he went down on the ice.
“I have no idea,” Avalanche coach Jared Bednar said when asked about the seriousness of Rantanen’s injury, according to ESPN. “I mean, he wasn't good enough to continue tonight as far as I know. We'll just have to re-evaluate him in the morning and see what happens.”
Colorado lost the game 3-1 and fall 7-1-1 this season.
The fifth-year forward had played in nine games this season. He has five goals and seven assists.
The 23-year-old Finland native played 74 games for Colorado last season and scored 31 goals and recorded 56 assists. He made the All-Star team last year as well.