Stefan, the Thrashers' first pick in their inaugural 1999 draft, was traded to the Stars on June 24. He returned to score his first goal and earn his first two assists of the season.
Atlanta was left with its first three-game losing streak of the season.
Marty Turco stopped 20 shots to help Dallas win for the ninth time in 10 games against Atlanta.
Turco gave up a goal to Steve Rucchin only 42 seconds into the game and then shut out the Thrashers until midway through the third period.