Lehtonen suffered the injury in the first period of Saturday’s game at Phoenix while stopping a shot by Coyotes defenseman Derek Morris on a play that was offside.
Lehtonen flew back to Dallas over the weekend and had an MRI.
“Kari will be out at least three weeks,” said Stars coach Glen Gulutzan. “We got some good news on that front as far as the MRI. It’s not a high groin pull or anything; it’s kind of a low strain, low pull. Those tend to heal a little better, so we’re looking at the three week mark before he can get going again.”
Lehtonen will miss at least eight games, including a five-game road trip.
Vernon Fiddler, who left Saturday’s game with a groin, will probably play tonight at Colorado.