Lehtonen became the highest-drafted Europe-born goalie in NHL history when the Atlanta Thrashers selected him No. 2 in the 2003 NHL Draft. That occurred after Lehtonen began the 2001-02 season as the backup with Jokerit in Finland's top pro league, but took the starting job, was named the league's top goalie and its playoff MVP after sparking his team to the championship.
After one more season in Finland, Lehtonen came to North America in 2003. He spent most of 2003-04 with Chicago of the American Hockey League but went 4-0-0 in a late-season callup, allowing five goals in four Thrashers wins.