O'Bryant is seeking a role in big picture

At the start of the exhibition season, there was a chance Patrick O'Bryant's long frame and offensive skills could land him a role as the Celtics' backup center.

There's still a chance, but yesterday, two days before the team opens the season against Cleveland, O'Bryant was practicing with the third team, with Bill Walker and Brian Scalabrine his company in the frontcourt.

Leon Powe and Glen Davis worked with the second unit as the backups for Kevin Garnett and Kendrick Perkins. Even if coach Doc Rivers believes O'Bryant has potential, he is comfortable going with players he knows from a season ago.

"It's just the other two guys are playing better," Rivers said about O'Bryant. "I don't need Pat to do anything. He's got to outplay the other two guys. It's like we tell him every day, there's a Patrick speed and a Celtics speed, and we're trying to get him to the Celtics speed all the time. But he's getting there."

Powe and Davis competed last year with P.J. Brown for minutes in the frontcourt, and then O'Bryant arrived over the summer.

"For me it's team," Powe said. "But I'm going to come out here every day competing. It don't matter to me. When you step on the court, I'm competing like I ain't got a spot. But as far as competition, it's good for the team, good for each other, because it helps you."

Rivers criticized Davis for poor performances early in the preseason. In eight games, Davis finished with the highest foul total (29) and was third in turnovers (14). But he took it as motivation, tying O'Bryant for second in rebounding.

"Doc has a way of casting his characters," Davis said. "He casts the big three for a starring role. I'm just a role-player. I'm just a guy on the side of this movie that we're shooting."

All the backup big men will figure out their roles as the season progresses.

"I'm trying to stay in the rotation," O'Bryant said. "Obviously we'll see how hard I've been working on opening night with who's out there and who's not, but I think I've been working hard."

At the ready
Despite not having played in any exhibitions, Sam Cassell said he's ready to play, making the likely point guard hierarchy Rajon Rondo, Eddie House, and Cassell.

The 15-year veteran said he's gotten all the reps he needs in practice and that the preseason is extra work when you've been around as long as he has.

"If you ask the guys about me, I'm playing well," said Cassell, who's put the same post move on everyone from Ray Allen to Austin Rivers, son of the coach, in post-practice goof-off sessions.

"Preseason at my age really doesn't mean [anything]," added Cassell. "Doc uses preseason to see what guys like Gabe [Pruitt] can do in extended [minutes]. To play four guards would be difficult"

Cassell said he's developed his rhythm in practice.

"If you watch our practices, we go after it," he said. "It's totally different than the games. But from practicing against these guys . . . I'm practicing against three All-Stars."

Many have tapped Cassell as one of the best candidates to eventually become a coach. Cassell says he's hearing that call, but in the end he always adds "one day."

Ringing in
Scot Pollard will be on hand for tomorrow night's ring ceremony, and the Celtics are working out the details to get Brown to town. With New Orleans tipping off at Golden State Wednesday night, new Hornet James Posey won't be in attendance, despite being a crucial piece on last season's title team.

Julian Benbow can be reached at jbenbow@globe.com.