Boston ? Paul Pierce never planned to leave Boston when he opted out of the final year of his contract. He just wanted to help the Celtics find him some teammates.

That decision became more important when a new challenge emerged to the Celtics’ hopes of repeating as Eastern Conference champions: the Miami Heat, with LeBron James, Chris Bosh and Dwyane Wade.

“I never thought once about leaving the Celtics,” Pierce, a former Kansas University standout, said Thursday when the team announced he had signed a four-year deal. “I wanted this team to get better, knowing we didn’t win a championship. … Also, I wanted to ensure that I had a chance to retire as a Celtic.”

Since losing the NBA finals in seven games to the Los Angeles Lakers, Boston re-signed Ray Allen, who will be 35 on Tuesday, to a two-year contract and gave free-agent center-forward Jermaine O’Neal, 31, a two-year deal. Kevin Garnett, 34, also has two years left on his contract. Those three, plus Pierce, 32, have a total of 54 seasons of NBA experience.

At times, the Celtics seemed too old to go far in the playoffs, especially when they went 27-27 in their last 54 games last season. But they surprised the doubters.

They overcame that underdog status, so retaining it doesn’t bother Pierce one bit.

“We relished that role this year,” Pierce said in a conference call. “It really doesn’t matter. You’ve got to show it on the court.”

On paper, he said, the Heat are the team to beat in the East.

“You’ve got to expect going into the season, when you’ve got the caliber of players they have that they’re going to definitely be the favorite, along with the Lakers,” he said.

The sting of the Lakers’ 83-79 win in Game 7 still lingers with Pierce, an eight-time All-Star in his 12 seasons, all with Boston.

“I think that’s one of the hardest losses for me that I’ve taken in my career and, as much as you want to put it behind you, it’s just tough,” he said. “I’ve pretty much thought about it every day.”

Before that series, Boston beat Miami in five games before upsetting Cleveland and Orlando.

Pierce’s old contract would have paid him $21.51 million next season.

The Celtics didn’t disclose the value of the new deal, agreed to last week, but the Boston Herald said it was $61 million.