Lehtonen spent the 2004–05 NHL lockout season with the Wolves, adjusting to the North American playing style. After playing 57 games for the Wolves, of which he won 38, posting .929 save percentage and 2.27 GAA, the Thrashers called him up, and Lehtonen started the 2005–06 season as the starting goalie for Atlanta. On the opening night of the 2005–06 season, Lehtonen suffered a serious groin injury when Florida Panthers centre Nathan Horton collided with him. The injury sidelined him for 35 games. Lehtonen was once again injured on April 6, 2006, when Tampa Bay Lightning forward Chris Dingman collided with him; Lehtonen tried to get to his feet, but sprained his ankle in the process.