WALTHAM, Mass. — The Celtics’ 2009-10 season officially tipped off Monday afternoon when the team held its annual Media Day.

More than 50 reporters flocked to the Celtics practice facility in Waltham where the players held interviews, signed autographs, posed for pictures, and did commercial spots.

And with that, the Celtics began their quest to reclaim the NBA title they feel like they never got a chance to defend last season because of injuries to Kevin Garnett (knee), Leon Powe (knee), and Tony Allen (ankle).

“It’s good to be back,” Celtics captain Paul Pierce said.

For extra motivation, the Celtics put up Banner No. 18 at their practice facility in Waltham. Someone hung it next to their 2008 championship banner. But this banner is blank.

Celtics coach Doc Rivers said that he doesn’t know who put it there, but Pierce said that it is there for one reason — “We have to fill it in.”

Pierce said the Celtics are the team to beat this season if they remain healthy.

“I’ve seen when we are healthy what type of team that we have,” Pierce said. “When we were a healthy team last year we were on a record-breaking pace. That’s just what I believe. … I have total confidence in this ball club. I just know that if we are healthy [winning the title] is going to happen.”

Rivers has no reservations about stating the Celtics’ goal this year.

“Our goal is to win it this year,” Rivers said.

“We have a squad that we hope to do great things with,” guard Ray Allen said. “We take every day and use those days to get better.”

Playing without forwards Garnett and Leon Powe, the Celtics lost to the Orlando Magic in the Eastern Conference semifinals last season. But Boston went out and bolstered its bench by signing four-time All-Star forward/center Rasheed Wallace, 6-foot-6 swingman Marquis Daniels who is coming off the best season of his career (13.6 points, 4.6 rebounds), 6-foot-9 forward Sheldon Williams who was the No. 5 overall pick of the 2006 Draft, and re-signing Glen Davis, who averaged 15.8 points and 5.6 rebounds in the playoffs last year.

As a result, the Celtics will be one of the deepest teams in the league this year.

“We’ve had a very good summer,” Rivers said. “As a staff and myself, we sat with Danny [Ainge] right after the year and I gave him a shopping list and he pretty much fulfilled it. … We got more size and added a backup for Paul and Ray who can play both positions — a versatile player in Marquis Daniels who is perfect in that role. We added more experience. We added more offensive touch and stretching of the floor with Rasheed, so yeah we did a lot of the things we wanted to do.”

“When you’ve got two guys like a Marquis and Rasheed who are starters coming off of the bench, that’s a big help,” Pierce said. “[Daniels] is going to give me a break. He is going to give Ray a break and who knows, he is a guy who if one of us goes down, he can fill in and start so that’s the value that he brings, knowing that he’s been a starter.”

Garnett’s surgically repaired right knee is a big concern entering training camp. Garnett says that is knee is “almost 100 percent” and that he has been medically cleared to practice with the team.

Age is also an issue. Wallace, 35, Allen 34, Garnett, 33, and Pierce, 31, are not getting any younger. Rivers said he would like to monitor their minutes this season so that they stay healthy and are not worn down when the playoffs begin, so he plans on playing the Big Three between 32 and 35 minutes per game.

But Pierce said he does not want to play fewer minutes if that means the Celtics will lose games.

“That doesn’t make sense to me,” Pierce said. “I’m trying to win every game we play. ... When I step out on the court and the other guys step out on the court, we play to win.”

Rivers could have a tough time making everyone happy with the amount of scoring opportunities that each person gets, but he will be the first to tell you that if the 2008 season proved anything, it is that the Celtics are unselfish and they are willing to sacrifice personal glory for the chance to win an NBA title.

“The bottom line is everybody’s here for one purpose and that’s to win,” said Rivers. “When you’re together in that way, I don’t think egos are much of a problem.”

With all of the new additions, Pierce said this Celtics team is better than the one that won the NBA title in 2008. But Rivers said a lot of other teams at the top improved as well.

“The top of the East is really tough,” Rivers said. “It’s going to be more physical. It is going to make it a more difficult season for all of us in the East … and that makes each team better.”

The Celtics training camp begins today at Salve Regina University in Newport.

roblee@projo.com