Miro Heiskanen's injury is worse than initially feared.

The Dallas Stars defenseman underwent knee surgery and is considered month-to-month, the team announced Tuesday, per NHL.com's Mike Heika.

Heiskanen was initially considered week-to-week with a lower-body injury after colliding with Vegas Golden Knights captain Mark Stone in last Tuesday's game.

Heiskanen has already been ruled out of the 4 Nations Face-Off for Team Finland.

The 25-year-old's recorded 25 points in 50 games this season while averaging a team-high 25:10 per contest.