Celtics coach Doc Rivers said before Game 7 that “I would be very surprised if we didn’t play extremely well.”

His suspicion was right.

But for only three quarters.

Boston played well enough to win for much of the night, but certainly not during the fourth, when the Celtics often do their best work.

Each of the Celtics’ marquee players crafted strong performances through three quarters, but all were clearly outshined by the Heat’s stars in the fourth, a period in which Boston was outscored 28-15.

Boston ultimately was undone in the fourth by defensive breakdowns, poor rebounding, offensive inefficiency and an inability to slow LeBron James, Dwyane Wade and Chris Bosh when it mattered most.

“We started throwing the ball away, they started beating us off the dribble,” Rivers said. “I was trying to push every button we had.”

There was Kevin Garnett (14 points, seven rebounds) missing a bank shot with the Celtics down six midway through the fourth. There was Ray Allen (15 points) missing the type of three-pointer that he has hit a zillion times, then missing another jumper with his team on the ropes.

And there was Rajon Rondo, terrific for three quarters, fumbling down the stretch: He missed a layup in transition, threw away a pass with the Celtics down seven and 3:42 left, and then had a jumper blocked by Bosh. Then Rondo kicked a ball within his grasp and missed another jumper.

Rondo’s overall numbers were very good: 22 points, 14 assists, 10 rebounds and just three turnovers. Meanwhile, Paul Pierce (19 points) shot just 2 for 8 in the second half after a 5-for-10 first half.

The Celtics shot 7 for 18 in the fourth and were outrebounded 12-5 in the quarter. Boston’s pick-and-roll defense, often a strength, faltered in the fourth. On one play, Wade split Mickael Pietrus and Pierce for a dunk.

Boston left Bosh too much space on the perimeter, though it never expected he would make three three-pointers.

“We needed Bosh’s guy [on defense] to help us slow down LeBron,” Rivers said.

Brandon Bass, who scored 14 of his 16 points in the first half, did decent work defensively on James early on but was ultimately and predictably unable to prevent him from producing another monster game.

“LeBron is the most unselfish superstar,” Celtics guard Keyon Dooling said afterward. “He’s great for our game. He IS our game. We’ve got to lift him up instead of tearing him down.”

The Celtics did so much right in the first half, leading by seven at halftime and finishing the half on an 18-11 run after Garnett left with his third foul and 6:50 left. But momentum swung in the third, as the Celtics committed five turnovers and missed 10 of 17 shots. The Heat quickly erased its halftime deficit, and the teams went into the fourth tied at 73 before Boston was simply overwhelmed in the fourth.

With Garnett and Allen poised to become free agents, this could mark the end for this exact Celtics nucleus. Dooling said that thought made him cry after the game — the first time he has done that since his father died three years ago.

“We have hearts of champions, and that will always keep us connected,” Dooling said.

Said Pierce: “There are a lot of emotions. It was a great opportunity to play with some of the greatest players of all-time. Kevin changed the culture of everything we did around here. He made everyone accountable.”