Erik Cole scored on a breakaway with 4:49 to play, and the Stars rallied to defeat the Minnesota Wild 4-3 on Saturday night. Lehtonen injured his head in a collision with the Wild's Erik Haula at 6:37 of the third period.
Haula was given a major penalty for charging - Lehtonen's helmet came off as he fell backward into the crossbar - and was ejected. Tyler Seguin scored with 26 seconds remaining in the 5-minute power play to tie the game at 3.
''Obviously, not good,'' Stars coach Lindy Ruff said. ''(Haula) went in hard, (Lehtonen) hit the crossbar hard. Likelihood, it's a concussion on just a dirty play. He should be suspended.
''Fourth-liner takes out our goaltender.''
The newly acquired Thomas stopped all six shots he faced for the win in his Stars debut in relief. The Stars acquired the 39-year-old goalie from the Florida Panthers at Wednesday's trade deadline, giving them another seasoned goalie.
''For me, Tim looked calm in there,'' Ruff said. ''He made some plays that maybe younger goaltenders wouldn't make. That was vintage Tim Thomas. He's lived it and he knows how to play.''
With the game tied at 3, Cole picked up a loose puck at center ice and Wild goalie Darcy Kuemper came out to stop him, but Cole's shot went past Kuemper and into the upper left corner. Cole also assisted on Seguin's tying goal.
''We fought hard all game long and kept battling back and were able to grab the lead,'' Minnesota coach Mike Yeo said. ''And then all of sudden you take a 5-minute major and another mistake after that and you find yourself down, so it's tough.''
Wild defenseman Keith Ballard had the turnover that led to Cole's goal.
''Bad read on my part,'' Ballard said. ''I don't know if I telegraphed it a little bit or I just kind of misjudged how high he was playing in the D zone. Obviously, the guy picked it off.''
Both the tying and winning goals were scored under a banner raised before the game to honor Mike Modano, the franchise's all-time scoring leader.
Seguin also had two assists, giving him four goals and four assists in two games. Jamie Benn and Alex Chiasson also scored for Dallas.
Haula scored in the first period and Charlie Coyle in the second. Kyle Brodziak gave the Wild their only lead in the third.
The Stars, eighth in the Western Conference, moved within three points of the seventh-place Wild. Minnesota had won five in a row.
Dallas also increased the lead over No. 9 Phoenix to three points.
''With the 5-minute power play right off the bat,'' Thomas said, ''it kind of allowed me to settle in and adjust and think the rest of it through to be like, 'Man, this would be a very important win to us.'''
Before the game, the Stars retired Modano's No. 9 in front of a Dallas-record crowd of 19,109. The Stars then scored just 31 seconds into the game.
Rich Peverley dropped a pass to Jamie Benn in the right circle for a wrist shot into the right side of the net.
''It's I think one of the biggest days in franchise history when you put a guy like that up in the rafters and then you cap it off with that type of game,'' Ruff said. ''I think they were pumped by watching some of the video, some of the Mike stuff. I know I was because there's some dynamic goals there you get to watch.''
Added Seguin: ''I think if anything that motivated me, that's a true character guy, a big leader, an incredible hockey player and now a legend. Watching his jersey go up and seeing that (1999) Stanley Cup again, I think the only thing it did was make me more motivated and I think the guys felt the same.''
The Wild tied it on Haula's short-handed goal at 16:32.
But the Stars regained the lead 4 seconds into a power play at 1:55 of the second period. That snapped a streak of 20 successful penalty kills by the Wild over eight games.
Kuemper, who was making his 16th consecutive start, had 34 saves. Lehtonen made 26 saves.
NOTES: Seguin raised his team-leading totals to 29 goals and 37 assists. ... Benn has three goals and seven assists in six games. ... Forwards Matt Moulson and Cody McCormick made their Wild debuts after being traded Wednesday from Buffalo. ... The previous attendance record was 19,099 on March 16, 2012. It was only the fourth sellout in 34 Stars home games this season. ... The home team won every game in the season series. Minnesota was 2-2 and Dallas 2-1-1. The Wild are 1-14-5 in Dallas.