BOSTON — In the seventh game of a largely forgettable Eastern Conference playoff series but with their season on the line, the young Boston Celtics cracked Sunday night.

The outcome was a deflating 87-79 loss to the Cleveland Cavaliers in a game that left TD Garden stunned. Yes, LeBron James was superb. His 35 points, 15 rebounds and nine assists were all but expected.

But after showing so much promise and poise through their unexpected playoff run, it was simply eye-opening to see the Celtics crack under the game’s heaviest pressure moment.

“The pain is part of the path,” said a disappointed Celtics Coach Brad Stevens. “Anytime it ends short of a championship it’s tough. When it ends, it’s painful, but that is part of the path.”

So one of the most unlikely runs to the NBA’s championship round fell one step short. The Celtics, in the end, did indeed need Kyrie Irving to get the job done. The excitement of Jayson Tatum, Jaylen Brown and Terry Rozier will mean brighter nights ahead, but the Celtics dropped their first home game (10-1) of the playoffs and now James heads to the finals for an amazing eighth straight season.

This win cements James as the undisputed, certifiably greatest Celtics-killer of them all. First in Miami and now in his second tour in Cleveland, he has frustrated, tormented and extinguished the championship hopes of a string of Celtics teams. He’s done it in five of the last seven Celtics playoff runs.

“We’ve been counted out for a long time this season,” James said, referring to the Cavs’ subpar regular season. “At that point I just said let’s get the most out of this season. I’m trying to squeeze this orange and get the most juice out of it that I can. It’s a treat.”

This game had the drama – if not the style points – that was missing in the first six games. The Celts jumped to a 12-point second-quarter lead but by halftime James finally started getting help from wildly inconsistent spare parts like Jeff Green (19 points) and Tristan Thompson (10 points, 9 boards). That helped the Cavs cut the deficit to 43-39 at the half.

The third quarter was a disaster for the Celtics. They specialized in bricking quick-trigger 3-pointers, made 5 of 20 shots and saw the Cavs settle into a comfort zone on offense. Boston lost its lead and trailed heading into the final quarter, 59-56.

That set up the doomsday scenario for any NBA coach. You never look forward to facing LeBron but you definitely fear him in a close game as the clock is melting away.

James kept his team in the lead with his shooting and passing wizardry but faced a formidable adversary in the 20-year-old Tatum, who actually threw down a dunk on the King, complete with a cheap chest bump to follow.

It was all great fourth-quarter NBA theater, the kind that should unfold with a ticket to the finals on the line.

But ultimately Boston’s awful 3-point shooting (7 of 39) and 29 percent second-half shooting did the team in. That couldn’t trump James, who played all 48 minutes and received just enough help down the stretch to get the job done.

“He’s unbelievable,” Stevens said. “We’ve played now until May 25th and May 27th the last two years and started in September, and that’s every day. Every day that you’re totally focused on (basketball). He’s gone past that eight straight times. It’s ridiculous. With the pressure, with the scrutiny, it doesn’t matter. It’s just unbelievable.”

Anyone who doubts the greatness of James is a fool. Sure he didn’t put away a vintage Celtic team and the Cavaliers may get swamped in the finals. But he owns the heart of a champion and is nearly impossible to knock out.

He’s now led his team to wins in 12 of his last 15 elimination games and won his last six Game 7s. His stat line for this series: 33.6 points, 9 rebounds and 8.4 assists.

“The best,” said Coach Ty Lue, when asked where this effort ranks for James. “He’s had a lot of gaudy games but a Game 7, in Boston, the history behind Boston, playing a undefeated team at home. Amazing.”

Lue went on to say doubting James “sells; negative press on the Cavs sells.” That’s an interesting take because the NBA wants to keep selling James as long as it can in as many deep playoff runs as he can muster.

As for the Celtics, they feel their day will come. Maybe it won’t arrive until James is no longer playing, or at least moves out of their path.

That may come as soon as next season if James leaves the Eastern Conference. It may also come when the Celtics arrive at this moment fully healthy and, even more importantly, remembering this night and the pain that will linger for a while.

Related Stories