Teams that win Game 1 are 485-220 (68.7 percent) winning a best-of-7 NHL playoff series, including 7-1 in the first round this season.
Game 2 is in Toronto Tuesday (7 p.m. ET; NBCSN, CBC, SN, TVAS).
"I thought it was pretty sloppy, the way we started," forward Tyler Johnson said. "It got better as the game went on, but I still think overall we were a lot sloppier than we normally are. We've got to make a few more plays tape-to-tape, not rush things like we were.
"At the beginning, I don't think we had our feet into it. I don't think we were battling or competing hard enough."
It wasn't the reason they lost. They gave up only one goal, a tip by Charlie Coyle at 18:52, on 15 shots on goalie Andrei Vasilevskiy. They would allow one more in the second, at 4:34 on the power play by David Pastrnak, and the game-winner by Brad Marchand 1:17 into the third.
But by then, the tone was set. And the Lightning weren't happy about it.
"The promising thing for us is I thought we really turned it around," Cooper said. "By no means did we lose that game in the first period, but we didn't help ourselves. When you give a team a lead and shrink the game to 40 minutes, it's going to be tough on you."
There were near-misses for the Bruins in that first period, saved by Vasilevskiy or the post. Pastrnak hit a post. Ondrej Kase had a breakaway chance stopped. The Lightning didn't generate much.
"We were obviously disappointed with the way we came out in the first," Hedman said. "You can say it's been a couple days since we played, but at the same time we expect more out of ourselves and we knew that wasn't good enough in the first. … That's on us in that room to be better come next game."
As the game went on, the team "played tighter, played together," as defenseman Mikhail Sergachev put it. It was heartening, showing the Lightning what they can do and how they can play as the series goes on against the Bruins.
That was the good news.
"Did things open up?" Hedman said, of the second and third. "We made a much better effort in the way we played, in creating chaos and creating opportunities for ourselves. That's got to be the norm going forward."
Or they risk starting every game from behind, a dangerous play against a team as dynamic as the Bruins can be. Which was why after Game 1, the Lightning vowed that they learn from their first 20 minutes on Sunday, and that they would be better going forward.
"There's no moral victories here," Cooper said. "There's lessons learned and to beat the Presidents' Trophy champions, we've got to play for 60."