WALTHAM, Mass. — The aging Boston Celtics are gearing up for what could be one final run at a title, and Ray Allen isn’t on the court.

The NBA’s all-time leader in 3-pointers and a member of the Big 3 that brought Boston its unprecedented 17th championship, Allen missed practice on Friday as the Celtics began their preparations for their first-round playoff series against the Atlanta Hawks. Allen, who has a sore right ankle, has not played in two weeks.

“I think he’s frustrated,” Celtics coach Doc Rivers said. “You want to go for it this year, and his body’s not cooperating. (And that’s) for a guy whose body’s always cooperated before.”

Allen, 36, was en route to another solid season in a probable Hall of Fame career when he injured his ankle in March and missed six games. When he returned, he began coming off the bench for the first time in his 16-year career so the Celtics could get Avery Bradley into the lineup.

But even with the extra rest, Allen lasted just five games before the ankle injury flared up again. He hasn’t played since April 10, missing the last nine games of the regular season.

“It’s always frustrating when you’re sitting out,” said Paul Pierce, who practiced Friday despite a sprained left big toe that briefly took him out of the regular-season finale on Thursday night. “Ray is one of the great competitors in the league. I’m sure it eats at him every single day he can’t practice and every single game he can’t play.”

Allen joined Paul Pierce and Kevin Garnett in a New Big 3 that won it all in 2008, their first season together, bringing the Celtics their first title since 1986. They returned to the NBA finals in 2010, losing to the Los Angeles Lakers in seven games.

Pierce is 34, and Garnett will be 36 next month and a free agent a month after that.

So this has been seen at their last chance at a title together.

“We don’t know what’s going to happen in the future,” Pierce said. “There’s definitely a sense of urgency there. A lot of things are going to be up in the air this summer. We want to try to get it done one last time.”

Rivers said he could sense the impatience — especially from his older players.

“They’ve got one or two chances the rest of their career and they don’t want to blow this,” he said. “But the body is the final decider. The clock is ticking on (everyone).

Rivers said Allen would not practice on Friday but might on Saturday, calling him probable for Sunday’s game but admitting he had no confidence in his prediction. He said he wasn’t sure how Allen would react to his longest layoff since missing the last 16 games of the 2006-07 season, before he was traded to Boston.

“His rhythm is not going to be good. He hasn’t played in two weeks,” Rivers said. “He hasn’t even run on the floor. If he’s ready to play, we’re going to play him. … I’d like to see him on the floor. Rhythm or no rhythm, if I tell Ray to stand in the corner, someone’s going to stand next to him. That creates spacing and that’s good for us.”