The Miami Heat suffered a crushing 110-97 defeat to the Boston Celtics in Game 5, cutting their series lead in the National Basketball Association Eastern Conference finals to 3-2, but center Bam Adebayo insists their confidence hasn't taken a hit.

Miami had been 3-0 up in the series before two straight losses raised the prospect of an unprecedented comeback for the Celtics.

No team has ever won a best-of-seven playoff series from 3-0 down but the Celtics head to Miami for Saturday's Game Six with the momentum.

Adebayo, who top scored with 16 points as Miami star man Jimmy Butler was restricted to 14 points, says the Heat haven't had their belief shaken by back-to-back defeats.

"Why would we lose confidence? When we started this journey, nobody believed in us. Everybody thought we were going to be out in the first round. Everybody thought we were going to be out in the second round. And now we are here one game away. For us, we've always had confidence, and that's not going to go away," he said.

Miami seemed to be in control when they blew out Boston 128-102 in Game 3, only to lose the next two.

Miami coach Erik Spoelstra was also dismissive of the impact of apparent change in momentum on his team.

"Who cares about mood? We have a gnarly group. I think so much of that is overrated. It's a competitive series," he said.

"You always expect things to be challenging in the Conference Finals. One game doesn't lead to the next game. Based on all the experience that we've had, it doesn't matter in the playoffs. It doesn't matter if you lose by whatever."

"We beat them by whatever in Game 3. It just doesn't matter. It's about collectively preparing and putting together a great game. We'll play much better on Saturday. That's all we just have to focus on right now," he added.

Playing better will certainly require finding more opportunities for Butler against a Boston defense that packed the paint effectively.