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’s123-112 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 1/2-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 victory “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.”

SUNDAY’S GAMES

Boston 100, Miami 98, 2 OT LA Lakers 105, Oklahoma City 96 New Orleans 91, Memphis 83 Detroit 104, Orlando 102 New York 106, Atlanta 104 Dallas 110, Phoenix 95 LA Clippers 96, Portland 83

TODAY’S GAMES - All times Central

Memphis at Philadelphia, 6 p.m.

Golden State at Toronto, 6 p.m.

Sacramento at Washington, 6 p.m.

Orlando at Brooklyn, 6:30 p.m.

Charlotte at Chicago, 7 p.m.

Indiana at Denver, 8 p.m.

Houston at Utah, 8 p.m.

Sports, Pages 18 on 01/28/2013