Lehtonen was injured in the third period Saturday night in a win over Minnesota when Erik Haula ran over him and knocked off Lehtonen's helmet.
Stars coach Lindy Ruff said Sunday that Lehtonen was "not doing that well" after saying after the game that his goalie likely had a concussion. The team didn't update Lehtonen's condition any further.
Tim Thomas, acquired in a trade with Florida at the deadline last week, replaced Lehtonen and got the win when the Stars scored twice in the final 10 minutes to beat the Wild 4-3. He's likely to start Monday night against Columbus, with Nilstorp backing him up.