WALTHAM, Mass. (AP) - All it took for Patrick O'Bryant to understand the significance of signing with the Boston Celtics was a look skyward at the team's training facility.

That's where the 17th NBA championship banner now hangs - instead of the white light shining on a blank patch of wall that once motivated players during practice.

"There's a lot of tradition here and you see all the banners in the stands and you see the newest one and it says '08'. I know that we're out here to put another one next to it and I'm just out here to do that," O'Bryant said.

O'Bryant, 22, joined his new team for a workout Thursday after signing as a free agent a day earlier - the same day free agent James Posey left Boston and signed a four-year, $25 million contract with the New Orleans Hornets.

O'Bryant, a 7-foot center, will be expected to help fill the void left by Posey, a 31-year-old veteran who has a reputation as a strong defender.

"I think I can definitely help with that. He was a very athletic type of guy," O'Bryant said. "He could guard a good range of people. I think I can probably come in and help with that kind of stuff, guard some of the bigger (players) if they're playing a lot of post-ups with a lot of the bigger threes."

Boston Celtics general manager Danny Ainge, who did not take questions about Posey during Thursday's media availability, said O'Bryant could benefit from his new teammates, including Paul Pierce, Kevin Garnett, Ray Allen and center Kendrick Perkins.

"I really think coming into our environment as a young player, with guys like K.G., Ray and Paul and Perk and Leon Powe, that he can grow fast and he can learn with our coaching staff and be a big contributor to the Celtics," Ainge said. "He has a great desire to live up to his potential and we're going to give him every opportunity."

O'Bryant showed that promise in 2006, when he was the ninth overall pick out of Bradley in the NBA Draft.

"I think Patrick was a very high draft pick and came out as a sophomore in college with a very bright future," Ainge said. "We had a lot of different big guys in for workouts and Patrick is the guy we really feel has the most talent. We think he has a great upside."

But O'Bryant acknowledged Thursday that his work ethic and intensity lacked in his first two seasons in the NBA with the Golden State Warriors, a high-powered offensive team. He appeared in just 40 career games with the Warriors, averaging 1.7 points and 1.3 rebounds.

He said he's looking forward to a change of philosophy where defense comes first.

"I might have come in and been a little immature and didn't really understand what the NBA was really about," O'Bryant said. "I guess the light turned on and I see that it takes a lot of work to be successful in this league. I think I'll help the club with a lot of defense, shot-blocking, on the guards and the big men and help guarding the post, basically."