BOSTON — A double-overtime victory against LeBron James and the defending NBA champions was difficult to celebrate for Paul Pierce and the Celtics.

News of Rajon Rondo’s season-ending knee injury spoiled the party.

“Everyone was really happy for the win,” Pierce said after Boston beat the Miami Heat 100-98 Sunday. “It brought a dark cloud in this room when you heard the news.”

When coach Doc Rivers told his players after the game, the joy of ending their six-game losing streak stopped, even if they had just outlasted the team with the best record in the Eastern Conference, which came in with a four-game winning streak.

Now the Celtics must try to keep winning without their leader, who suffered a torn anterior cruciate ligament in his right knee late in Friday night’s 123-112 loss, also in double overtime, in Atlanta.

They won Sunday after Pierce’s 22-foot jumper with 31 seconds left gave them a 99-98 lead.

But making the playoffs got harder as the Celtics try to hang on to the eighth and final postseason spot in the conference with a 2 ½-game lead over the Philadelphia 76ers.

“Obviously, the Rondo news is pretty tough. I knew it before the game,” Rivers said. “I just didn’t think it was any time to tell any of our guys.”

This game was the first in Boston for Ray Allen since he left the Celtics after five seasons and signed as a free agent with Miami. He scored 21 points.

Kevin Garnett had 24 points and 11 rebounds, and Pierce added 17 points, 13 rebounds and 10 assists for the Celtics.

Rondo’s injury “puts this team and the rest of the guys in a position to be ready to step up,” Pierce said. Sunday’s win “was a perfect example. We showed we are capable.”

James had 34 points for the Heat, whose winning streak stopped at four.

“As much as I’ve been a rival with Boston over the years, I never want to see anyone go down,” James said. “It’s terrible, not only for their team but for the league.”

After Pierce’s basket, James had a chance to put the Heat ahead but missed a 12-foot jumper with 6.8 seconds to go from the left with defender Jeff Green jumping out at him. Pierce got the rebound and was fouled by Shane Battier.

He sank the first shot. Then, as a fan shouted “This one’s for Rondo,” he missed the second.

Miami had one last chance, but Battier missed a long jumper at the buzzer.

“They defended that very well,” Heat coach Erik Spoelstra said. “There are about three different options to it, four different options to it. They defended each one of them.”

The Heat also could have won in the first overtime, but Dwyane Wade, who had 17 points, also missed a long jumper as the buzzer sounded. They had led 93-89 after consecutive baskets by James, but Garnett hit a layup with 1:45 remaining and a shot from the right baseline with 1:14 to go.

Boston could have avoided the first overtime when Pierce inbounded from behind his backboard with two seconds left to Jason Terry. But Terry’s shot from the top of the key was short. The Heat had tied it on a 3-pointer by James with seven seconds remaining in regulation after Allen missed a 3-pointer from the left corner with 15 seconds to go.

Rivers did not mention Rondo’s injury in his meeting with reporters about an hour before the game, but Courtney Lee started in his place. Doctors decided to keep Rondo out of the game after he went through his normal pregame routine but complained of pain that he thought was in his hamstring. An MRI was done, and team physician Dr. Brian McKeon learned the results during the game.

Allen was part of the Big Three with Pierce and Garnett starting in 2007-08. In their first season together, Boston won its 17th NBA championship. He played against the Celtics once before this season, a 120-107 Heat win in Miami in the opener.

The crowd gave Allen a standing ovation when highlights of his career with the Celtics were shown on the video board above center court during a timeout with 5:33 left in the first quarter. At the Miami bench, he raised his left hand in recognition.

“When I saw it, just all those emotions came streaming back from all the great things we did here,” Allen said. “I’ll always be a Celtic in my mind.”

LAKERS 105, THUNDER 96

LOS ANGELES — Kobe Bryant had 21 points, 14 assists and nine rebounds, Steve Nash added seven of his 17 points in the final 5½ minutes, and the Lakers held off the NBA-leading Thunder.

Pau Gasol scored 16 points in a reserve role as the Lakers picked up the most impressive victory of their thoroughly unimpressive season, coolly maintaining a small lead down the stretch of their second straight win after a four-game skid.

HORNETS 91, GRIZZLIES 83

MEMPHIS, Tenn. — Ryan Anderson scored 22 points for New Orleans, connecting on seven 3-pointers.

Anderson was 7 of 13 from the field, all of his attempts coming from outside the arc. Fellow reserve Jason Smith was 5 of 7 from the field for 16 points, and Anthony Davis was the only New Orleans starter to finish in double figures with 10 points.

PISTONS 104, MAGIC 102

ORLANDO, Fla. — Brandon Knight had a career-high 31 points, including five 3-pointers, and Greg Monroe scored 17 to lead Detroit to the road win.

The Magic had a chance to tie the game in the closing seconds after a missed free throw, but missed three putback attempts.

Mavericks 110, Suns 95

DALLAS — Shawn Marion scored 18 points in his 1,000th game, Dirk Nowitzki also had 18 while passing Allen Iverson on the career scoring list and the Dallas Mavericks beat the Phoenix Suns.

It was the 10th straight home win for the Mavericks against the Suns, who haven’t won in Dallas since March 14, 2007.

Nowitzki passed Iverson for 18th on the NBA’s scoring list with a third-quarter jumper that stopped a Phoenix run, and his 3-pointer early in the fourth started a spurt that put Dallas in control for good.