From the moment they were assembled, they played with an expiration date hanging over their heads. Three years, maybe five. There’s a finite amount of time in this game and for three superstars in the latter stages of their career that time is precious.

“There’s definitely a sense of urgency there,” Paul Pierce said. “You try to get a banner one last time, you try to go out with a championship. We feel we have all the pieces, we have all the ingredients to do it. A lot of things are going to be up in the air this summer.”

They are realists, all of them. From Pierce to Kevin Garnett to Ray Allen to Doc Rivers, they all know that there will be no more reprieves. Playing together has been a basketball paradise. Together, they complement each other beautifully and they all know what it is like to be adrift in the vast mediocrity of the NBA.

It was almost broken up at the trade deadline in March, a thought Rivers termed, “scary.” They were almost broken up in 2010 too and every summer there seemed to be a fresh round of Rajon Rondo rumors, who is most certainly a part of this close-knit collective. 

Still, they persevered. They won a championship in their first season and could have won a second if Garnett’s knee had cooperated. They made the finals in 2010 when they were ticketed for the morgue, which extended the run a few more years. This, however, is it.

“They know,” Rivers said. “The clock’s ticking.”

They will begin their last stand against a Hawks team that is similarly constructed. Five years ago, the Celtics were the overwhelming favorite and Atlanta was the up-and-coming squad with something to prove. In a league that churns on constant upheaval, both teams have stayed remarkably intact. The Celtics have made the deeper runs, but the Hawks have also been a consistent playoff presence, albeit with a second round ceiling.

“Both teams are playing for survival as a group,” Rivers said. “I really believe that. If we win this series and keep moving on, we’re going to want to build on this team. If they win, something can happen to us. If we win, you may not see that same Atlanta team. Both teams are playing for survival.”

There will be changes this summer, without question. The Celtics have only four players under contract for next season and Garnett and Allen are both free agents. No one is really certain what will happen with those two, which makes Allen’s ankle injury all the more problematic. He revealed on Saturday that he needs surgery because of bone spurs that have formed in his ankle. 

“Yeah, no question. If I could, I would get surgery tomorrow,” Allen said. “If this was earlier in the season I probably would have had surgery on it.”

Allen’s not going out with a fight. He had a second cortisone shot and he’s making the trip to Atlanta, although it’s highly unlikely he’ll be able to play Game 1, beyond that is anyone’s guess. 

“I think he’s frustrated. I think he’s extremely frustrated,” Rivers said. “And he should be. He wants to go for it this year and his body’s not cooperating for a guy whose body has always cooperated. For him, he’s done everything right. He came into camp in the best shape of anybody. He broke a record on the treadmill and now his foot hurts and he can’t do anything about it. That bums him out.

“The veterans, these things mean so much to them now,” Rivers continued. “They got one, two chances at this the rest of their careers. He doesn’t want to blow this, but the body is always the final decider.”

In this hurried season, developments have shifted rapidly. The Celtics were old and cooked, then suddenly revitalized only to fall apart again. Without warning they were reborn a final time as a smaller, sleeker instrument of defensive destruction.

The catalyst has been 21-year-old Avery Bradley, who was all of five years old when Garnett made his NBA debut. With Bradley starting in place of Allen, the defense is once again the best in the league and the offense has an updated look with a dynamic new weapon. Allen’s long distance running has been supplanted by Bradley’s short sprints to the basket via the backdoor. 

Mickael Pietrus, the effervescent import has given the Celtics a charismatic version of James Posey. Long and tough with an unbreakable lack of conscience behind the 3-point line, Pietrus has helped add a layer of versatility to a team built on a conventional approach.

“We have two guards who can guard multiple positions,” Rivers said of Pietrus and Bradley. “That’s really something I don’t know if we’ve ever had.”

With those two playing more prominent roles and Garnett’s move to center, the Celtics complied a 24-10 record after the All-Star break. They are still longshots, but they have more than just that proverbial puncher’s chance on their side now.

“There was more pressure being the Number One seed, winning the most games going into the playoffs that year, than there is now,” Pierce said. “Everything we do now is going to take people by surprise. If we win first round, second round, go to the Eastern Conference finals, there’s pretty much no pressure on us. We like that role.”

From favorites to spoilers, the Celtics have come full circle. All things considered, it’s been a rather remarkable ride and the final lap begins Sunday.