The Islanders won the game, 3-2 in overtime, to force Game 7 on Friday night in Tampa, Florida.
"It sucks to lose Kuch like that early," Lightning captain Steven Stamkos said of Kucherov, who leads all playoff scorers with 27 points in 17 games. "Guys battled. We played short the rest of the game."
Kucherov continued his shift, throwing a check on Islanders forward Mathew Barzal. He eventually doubled over in pain, and then left the ice for the trainers' room. Kucherov did not return, having played only 46 seconds in the Lightning loss, leaving the ice just 2:22 into the game.
Tampa Bay's players and coach seemed to believe it was the cross-check that injured Kucherov.
"The play on Kuch, the ref doesn't see it. Probably a guy looking for a cheap shot there," Stamkos said. "I think [the referees] said they didn't think it was malicious. They don't have a chance to look at it on replay like we do."
Tampa Bay coach Jon Cooper said that intent wasn't criteria for it to be a penalty.
"It would be weird to say it wasn't malicious," Cooper said. "I don't think that's in the rule book, that cross-checks have to be malicious. I just know it happened really close to one of the officials, and he didn't see it."
Adding insult to the injury, it was Mayfield who tied the game at 11:16 of the third period, before Anthony Beauvillier ended it at 1:08 of overtime.
The Lightning, who went 3-for-6 in Game 5 on the power play, were 0-for-2 in Game 6. That included a power play with less than six minutes remaining in the third period, with a chance to take the lead.
"We were short a player, so that probably didn't help out," said Cooper, who otherwise was content with how players like Anthony Cirelli helped fill the hole left by Kucherov on his line with center Brayden Point -- who scored a goal in his ninth straight playoff game, one short of an NHL record -- and winger Ondrej Palat.
"It's next man up. These guys, this collective group, there wasn't panic in the group at all," Cooper said.
The Lightning return home with Kucherov's status unclear. Of course, they're familiar with life without their star winger. Preseason surgery on his hip led Kucherov to miss the entire regular season before he returned for Game 1 of the first round.
"We've been down this path for four months. We won games with him and we won them without him," Cooper said.