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, who is tied for the league lead with 13 wins and had a 2.34 GAA and a .926 save percentage, flew back to Dallas over the weekend and had an MRI.
"Kari will be out at least three weeks," Gulutzan said. "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. To compensate, the Stars called up goaltender Richard Bachman from the Texas Stars of the AHL.
Bachman has played 15 games for Texas this season, posting a 7-5-1 record, 3.13 goals against average and .887 save percentage. He played in one game for Dallas last season, stopping all four shots he faced in 9:35 of playing time against Phoenix.
Center Vernon Fiddler, who also left Saturday's game with a groin injury, played and had an assist against the Avalanche.
Mark Stepneski covers the Stars for ESPNDallas.com.