"I don't think you can ask for anything better than what happened to Kari," Atlanta coach Don Waddell said. "He was on top of his game."
Lehtonen, who got his first shutout this season and eighth in his career, lost his first four decisions before being placed on injured reserve. He missed 16 games, dating to Oct. 18, before being recalled from AHL Chicago earlier Saturday.
"We were better prepared, and we played better as a team," Atlanta forward Bobby Holik said.
Wade Dubielewicz made just his second start for the Islanders and stopped 26 shots. Dubielewicz, subbing for Rick DiPietro who had made 18 straight starts, gave up seven goals to Toronto on Oct. 11 in his only other starting assignment. Both outings came after the Islanders played the rival New York Rangers.
The Islanders haven't scored more than two non-shootout goals in 12 games and are 5-6-1 in that span. They have scored at least three goals only eight times.
"We're so spread out, we don't give ourselves a chance to play together," Islanders captain Bill Guerin said. "It's tough for a goalie to play 5-on-0."
It only took Kovalchuk 34 seconds to score his NHL-leading 21st goal and give the Thrashers the lead. He moved down the left boards, around defenseman Radek Martinek, and sent a shot from the left circle past Dubielewicz.
"It was an all-star goal," Islanders forward Mike Sillinger said. "We had all our coverage. He made a great move."
Kovalchuk said the early goal was part of the Thrashers' plan.
"We knew they played a goalie that doesn't play a lot," Kovalchuk said, "so we wanted to jump on them early."
Atlanta then scored twice in a 50-second span to go up 3-0.
Boulton got his first of the season on an uncovered rebound of Jim Slater's shot at 6:12. At 7:02, Hossa beat Dubielewicz on a break-in with a wrist shot high to the blocker side.
"It's tough, Dubielewicz said. "It just feels like we got behind the 8-ball, and I got rattled out there a bit."
The Islanders didn't register their first shot on Lehtonen until over seven minutes had been played.
"I thought we would be ready to play," Islanders coach Ted Nolan said. "I was wrong. We just weren't prepared right from the beginning. We underestimated their team speed. They really took it to us the first 5-10 minutes of the game."
Kozlov capitalized on New York's poor clearing attempt when he ripped a shot past Dubielewicz with 13 seconds left in the period.
Halfway through the second, the Islanders were being outshot 6-1. Dubielewicz kept the deficit at four when he stopped Hossa on a breakaway with a right pad save at the midway point.
The Islanders came close to breaking Lehtonen's shutout bid with 9:30 left during a power play, but Marc-Andre Bergeron's shot from the right point hit the left post.
"It's nice to be back," Lehtonen said, "and it's nice to see everybody play well."
Islanders C Shawn Bates, who made his season debut Wednesday after being out since January with abdominal injuries, sat out due to a sore hip. ... New York has three power-play goals in its last 43 chances. ... Thrashers general manager Don Waddell is 12-7 since taking over as coach for the fired Bob Hartley. ... Lehtonen won his two AHL starts, posting a 1.93 goals-against average and a .934 save percentage. ... New York is 11-3 against the Eastern Conference's playoff teams from last season. The Thrashers are the only team in that group the Islanders have yet to beat this season.
Copyright by STATS LLC and The Associated Press
Arena: Nassau Veterans Memorial Coliseum Location: Uniondale, New York
Referees: Bill McCreary, Ian Walsh Linesmen: Derek Nansen, Jay Sharrers