Lehtonen was drafted second overall in the 2002 NHL Entry Draft by the Atlanta Thrashers after an outstanding 2001–02 season in the Finnish SM-liiga. Lehtonen was the backup goalie for Jokerit, but captured the starting position and won the Urpo Ylönen trophy, given to the best goalie of the season, and the league's playoff MVP award, the Jari Kurri trophy. Lehtonen stayed with Jokerit for one more season before moving to North America.