WALTHAM, Mass. – Time was against these Celtics before.

Father Time.

When Kevin Garnett and Ray Allen brought their 30-something bodies to Boston in 2007, they joined Paul Pierce as a potent All-Star trio. But each year, observers talked about a small window of success for the aging Big Three.

It’s not closed yet, not even after the Celtics fell behind 2-0 in their playoff series with the flashier, younger Miami Heat.

“I think every year you have an opportunity,” Allen said Thursday. “So, I don’t look at it like it’s ‘The Last Hurrah.’ When I got here in ’07, everybody was saying, is this your chance, this year? And it’s four years later.”

And the Celtics still have a shot at their 18th NBA title.

Before that can happen, though, the Big Three must recover from the first 0-2 deficit in their 11 playoff series together.

The effort — and Coach Doc Rivers said it must be more energetic — begins Saturday night at home with Game 3 of the best-of-seven Eastern Conference semifinal series.

“Forget all the X and O stuff,” Rivers said. “I really thought Miami played harder in a lot of ways — all the loose balls, they finished at the basket (going) through us, they got three-point plays (that) we didn’t.”

The Heat won the opener 99-90 as Dwyane Wade scored 38 points and LeBron James added 22 — with James Jones helping with a surprising 25. They also took Game 2, 102-91, behind 35 points from James, 28 from Wade and 17 from Chris Bosh.

On top of that, the Celtics came out of that game battered.

Pierce hurt his Achilles’ tendon, Allen suffered a chest bruise and Rajon Rondo had a stiff back. Rivers expects all of them to be better on Saturday night. And, he said, Shaquille O’Neal probably will return for his first playoff game after his sore right calf improved.

What can he contribute after being sidelined for all but one game since Feb. 1?

“I have no idea,” Rivers said, “and I don’t say it to be funny. I just know he’s going to be big.”

Having the 7-foot-1, 325-pound O’Neal clogging the middle should slow Miami’s drives to the basket. The Celtics also must be more aggressive.

“You’d be amazed how often we did drive,” in the first two games, Rivers said, “but we drove and tried to make some crazy shot instead of going through bodies.”

That could have drawn more fouls. Instead, the Heat have taken 14 more free throws in each game and outscored the Celtics by 22 at the line.

The Heat also have shot better from the field. Allen led the Celtics with 25 points in the opener, but took only seven shots in the second game and finished with just seven points.

“I don’t know if it’s just me taking more shots as much as it’s just more offensive stuff being run for me,” he said. “That way, we get Wade involved more in defensively guarding me. Same thing with LeBron. I don’t know if we put enough pressure on either one of those guys defensively. And, I think, offensively they had their way.”

The concise analysis is simple, according to Glen Davis. The Celtics haven’t played their game — driving hard to the basket and moving the ball on offense and rotating on defense.

“Nobody played the way they were supposed to play,” Davis said. “You can coach as much as you want. It’s the guys out there that have got to make it happen.”

A theme often expressed in the playoffs is that the series doesn’t start until the road team wins a game. Rivers isn’t buying that.

“I think the series has started,” he said, “and we’re down 2-0.”

Wade had a different take.

“The series is just starting,” he said. “We’ve got to go up there like we would if we were starting on the road, and be like, ‘Let’s go get one right now.’ So our focus is on Game 3 and making sure that we’re even better.”

On their second of three days off between games Thursday, the Celtics watched film, walked through plays and worked out lightly. John Havlicek, who won eight titles in 15 seasons with Boston before retiring after the 1977-78 season, looked on from the sideline with 17 title banners hanging from the gym walls.

“I said, ‘which one of those banners were you 0-2?’ ” in the series, Davis said, “and he said the one that stood out to him was the 1969 (one), when they were down 0-2 and came back to win it in Game 7 against the Lakers. And he was just saying ‘it’s going to take everything in you to fight and claw back to get back to that 2-2.’ But then it’s going to take something special to finish them off.”