Despite LeBron James’ epic performance, despite their offensive shortcomings Thursday night, despite being crushed on the boards by 20 over the past two games, the Boston Celtics and their collection of stars are not particularly susceptible to self-doubt.

And so, the Celtics, 10-9 against the Heat over the past two seasons, say they will enter Game 7 with the confidence expected of a team whose nucleus won a championship together in 2008.

“We’ve been the underdog all year long,” said forward Paul Pierce, who shot 4 for 18 in Game 6. “We’re going into Game 7 the underdog. We’re right where we want to be. This is as hard as it gets. And I think we are prepared for it.”

Said center Kevin Garnett: “There are a lot of confident guys in here, a lot of guys who have been through Game 7s. A lot of experienced guys. We’re going to lean on that. There’s a bunch of fighters in this locker room.”

Though James torched the Celtics in Game 6, coach Doc Rivers said he is not inclined to change his defensive approach at the start of the game.

“We’re always prepared to do different things, but we don’t feel like we need to,” he said. “If somebody is going off, we’re ready to make a change if we have to. … We can’t assume he’s not going to score 45 again.”

Rivers said if the Heat wins the series, “that Game 6, you’ll always remember. If we win, Game 6 will be just another great game.”

Rivers said other than “Tiger Woods the last two or three years, no athlete is under the scrutiny LeBron is under.” He said he “doesn’t know what else he can do” to silence critics. “He’s one of the most powerful players to ever play the game. Maybe it isn’t enough. I don’t know.”

Celtics players were irked that they did not do more to impede James.

“He was comfortable all night,” said Rajon Rondo, who had 21 points and 10 assists but also seven turnovers. “We didn’t get into his airspace. I have to do a better job as far as taking care of the ball, getting into my sets. I have to make it harder for him. It was too easy.”

The problem defending the Heat, Rivers said, is this: “In the playoffs, as good as your stuff is, the guy that can create his own shots makes it very difficult. Even when the set breaks down, you can always throw it to LeBron or Dwyane Wade and they can get a shot.”

Garnett was limited to 12 points Thursday (on 6-of-14 shooting), and Rivers said, “We have to do a better job of getting Kevin the ball in the right spots. They threw him out of his rhythm, and we threw him out of his rhythm.”

The winner of Game 7 will fly to Oklahoma City on Sunday, with Game 1 of the Finals on Tuesday night.

Rivers said Thursday that his team “is not just going to pack for Sunday. They’re going to bring suits for Tuesday and Thursday.”

Rivers said Friday he intended no message when he said that: “What else am I going to tell them? We have to pack.”

The Celtics are 5-1 after losses in these playoffs, with their only back-to-back defeats coming in the first two games of this series. Against Atlanta and Philadelphia, the Celtics followed their five losses with wins by an average margin of 10.6 points.

“We got in the locker room after the game [Thursday], and everyone talked about being better,” Boston guard Ray Allen said. “There was a great sense of disappointment among everybody. Anger. But I know my guys will be ready next game.”