The Thrashers received defenseman Ivan Vishnevskiy and a fourth-round pick in the 2010 draft. Atlanta said Vishnevskiy, 21, is expected to report to AHL Chicago.
Atlanta traded leading scorer and team captain Ilya Kovalchuk to New Jersey on Thursday for three players and a first-round pick.
Lehtonen, a native of Finland, posted a 94-83-17 record in 204 games with Atlanta, setting franchise records for wins and games played by a goaltender. He has not played in the NHL this season following two back surgeries.
Lehtonen was recalled from a conditioning stint with AHL Chicago on Feb. 7. He posted a 1-1-2 record with a 2.67 goals-against average in four games with Chicago.
Lehtonen's last NHL action came on April 11, 2009, when he made 33 saves in a win against Tampa Bay. He was Atlanta's first-round pick in the 2002 draft.
The Thrashers have relied on the combination of veteran Johan Hedberg and 22-year-old Ondrej Pavelec in goal this season.
Vishnevskiy, a native of Russia, was a first-round pick in the 2006 draft. He has two assists in five career NHL games with the Stars but was scoreless in two games with Dallas this season.
He had eight goals and 16 assists in 51 games with AHL Texas, and had two assists in the AHL All-Star Classic on Jan. 19.