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 post-season 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 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 centre 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."
Allen entered the game about a minute after the tribute and was booed when he touched the ball. The boos came down again when he took two free throws — missing the first, making the second — three minutes into the second quarter.
Allen went back into the game with 30 seconds left in the first half with the score tied at 43 and made a layup with 16 seconds to go. But Pierce sank a driving layup with 4.3 seconds remaining to tie it 45-45 at halftime.
"What happened to Ray is never good for this franchise," Pierce said, "but it wasn't about Ray. It was about us playing the defending champs."
It was Miami's first game in Boston since it won Game 6 of the Eastern Conference finals behind James' 45 points.
Early in the second half, James hit a pair of 3-pointers from the same spot on the left, putting the Heat in front 51-48. The Celtics rallied and led 64-61 with 1:49 remaining in the third quarter. But the Heat scored the last seven points of the period on a jumper by Norris Cole, a free throw by Allen and two 15-footers by James and led 68-64 going into the fourth.
Notes: James and Chris Bosh each had 16 rebounds for the Heat. The last time the Celtics played consecutive double-overtime games was on March 11 and 13, 1951.