BOSTON – Doc Rivers sat down recently for dinner with his Big Three and discussed an old subject: their age.

With Kevin Garnett and Ray Allen signed for just one more season, the coach of the Boston Celtics knows he might need a table for two with Paul Pierce a year from now.

“I always press on them, more than anyone, how small the window is for them,” said Rivers, hoping for one more championship before the rebuilding begins. “They understand that.”

Chances are tonight’s draft won’t provide much, if any, help toward winning that title.

The Celtics have the 25th and 55th picks in what is widely regarded as a mediocre draft. With only six signed players, there is room on the roster, but General Manager Danny Ainge is realistic.

“The 25th pick of the draft probably is not going to help us immediately,” he said Wednesday, “but there are some players that we think can fit our roster, that can fit in with the personality of the team, that have a work ethic that can make our team better in practice and that can just add depth to our roster.”

Not exactly what the Big Three would like to hear, but Ainge is hopeful he can sign free agents for low salaries. He did that last year with Shaquille O’Neal, who contributed early before injuries sidelined him and contributed to his retirement.

“He had given us every indication in the last couple of months of the season” he would retire even though he was signed through next year, Ainge said. “Then when he really couldn’t play, I think we would have been surprised had he not retired.”

The departure of O’Neal and Nenad Krstic, who signed a two-year deal with a Russian team, leaves the Celtics short on size.

Center Jermaine O’Neal played the last eight games after fracturing his left wrist in Game 1 of the first round but opted against surgery.

“He has every desire to come back and play, and didn’t like the way it finished this year and doesn’t want to end his career on that type of note,” Ainge said.

The Celtics lost to Miami in five games in the second round.

The only Celtics with contracts for next season are Garnett, Pierce, Allen, O’Neal, Rajon Rondo and the 2010 first-rounder, Avery Bradley. Forward Jeff Green, obtained Feb. 24 from Oklahoma City, is a restricted free agent the Celtics want back although he struggled.

“He’s integrated in our culture,” Rivers said. “I think it was a culture shock for him at some of the pace we practice at and stuff like that. He’s got skill.”

Rivers, signed after the season to a five-year contract extension through 2015-16, expects to have Rondo back healthy at point guard. He played the last two games against Miami after dislocating his elbow in Game 3.

“He’s not 100 percent yet. He’s probably about four weeks away from playing basketball full speed,” Ainge said on the Celts’ practice floor as Rondo ran on a treadmill on a balcony directly above. “Rondo’s a tough kid. He was in a lot of pain and was able to give us a good effort.”

Rondo was taken with the 21st pick in the 2006 draft, just four spots better than the Celtics’ first-round pick this year. And Tony Allen, chosen with the 25th pick in 2004, was a solid contributor during his six seasons with the team.

But Bradley is the only one of the six players drafted by the Celtics the past three years still with them.

Ainge is hoping for better luck.

“Sometimes we look to add more character, more toughness, more shooting, better ball handling, better decision-making, better upside,” he said. “All those things come into play every time we draft. And so rarely are those characteristics in one player. If they are, his name is Michael Jordan.

“This year I think we’ll be able to add some good pieces, but we’re not adding any starters.”

They still have plenty of motivation, especially the Big Three.

The Celtics won their 17th NBA title in 2007-08, their first season together. They came very close in 2009-10 when they lost to the Los Angeles Lakers in seven games. This year they didn’t get past the second round.

“It’s funny,” Rivers said. “You lost a Game 7 last year and I thought the disappointment this year was more (to them) in some ways.”

He had to wait to meet with them until he could talk following postseason throat surgery to remove a non-cancerous polyp.

“I didn’t talk for 11 days,” Rivers said. “I don’t think I was a good patient. I was talking a little bit sooner than I probably should have, but overall I feel great.”