LeBron James is basketball's ultimate test and the Miami Heat will have to be "near perfect" to defeat the Los Angeles Lakers in the NBA Finals, admits Jimmy Butler.

The Heat defeated the Boston Celtics 125-113 in Game 6 of the Eastern Conference finals on Sunday to clinch a 4-2 series triumph.

It marks the first time Miami will contest the NBA Finals since making the last of four in a row in 2014, which marked the James' final season as a Heat player.

Butler is under no illusions as to the size of the task the team faces but insists the Heat still have another level to reach.

"The main key, and it's been like this for a very long time, if you want to win, you're going to have to go through a LeBron James-led team," said Butler, who had 22 points and eight assists. 

"At the end of the day, that's what it normally comes down to. That's what we got to focus in on. 

"Obviously you can't focus in on him because he has so many really good players around him, but you're going to get the same test over and over again until you pass, and that test is LeBron James.

"I said it time and time again. I believe in our group. I know that we can still win four more. We're not satisfied. We're not complacent. 

"We know we've got a really good team to go up against. We can enjoy it for the night. We said this from the beginning of the year."

Asked how Miami have fought their way past teams who were seeded higher in the East, Butler replied: "Together. That's how we were able to do it. I don't look at five-seed or one-seed. 

"It's all about who is playing the best basketball at the right time.

"I feel like we're still yet to play our absolute best basketball, but along the way, we realise what we have to do moving forward.

"So we have to be near perfect to beat the Lakers. We're capable of it. But we're not worried about no five-seed or one-seed. We're worried about us and being the best Miami Heat team that we can be."

Bam Adebayo was a talisman for the Heat, putting up 32 points, 14 rebounds and five assists to dominate the Celtics just a couple of days on from taking the blame for Miami's loss in Game 5.

"I let my team-mates down again in Game 5. You know, I just had to realign myself and who I really want to become," he said. "I showed that tonight. You said you wanted to see me be a scorer in the fourth, well, there you go.

"It's just one of those things, you know, coaches they want me to be better and they want me to do great things. That's the first time I got plays, back-to-back, back-to-back. 

"Being in that moment and embracing it and taking over is a great thing for me. But at the end of the day, we got four more. Our business isn't done yet."

Celtics coach Brad Stevens conceded his team did not get a good enough grip on Adebayo.

"I thought today after we had the lead, Adebayo, and credit all of them, but Adebayo deciding he's just going to drive the ball put us in a real bind with the shooters around him," he said. 

"And their physicality is something that I'm not sure that we probably talked about enough. 

"They're strong, they're physical, they're tough and, him in particular, dominated that fourth quarter. Even the plays where he didn't score, his presence was so impactful and it put us in a real bind with the ability to guard him."