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 2.5-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.

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.

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

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.

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.