"We have to do a lot better job of defending," Tavares said postgame, according to TSN's Mark Masters. "Letting seven in at home is just unacceptable."
Tavares, who scored his first goal of the season in the opening frame, knows his club needs to be better.
"We just didn't execute at a very high level and they did," he said. "We just didn't do the right things that are going to consistently lead to good things and lead to results. That effort tonight isn't going to win us many hockey games."
For the third time in four home games this season, the Leafs surrendered the first goal in the opening 2:30 of the contest.
The Lightning struck for four first-period goals on Leafs netminder Frederik Andersen, including a pair on the power play. It was the first time in his career that Andersen has allowed four goals in the opening stanza of a regular-season matchup, according to Sportsnet Stats.
Toronto is now 1-2-1 at Scotiabank Arena to start the campaign. The Leafs will travel to Detroit to face the Red Wings on Saturday night before hosting the Minnesota Wild next Tuesday.