NEWPORT (AP) - The transition from college star to NBA rookie has been a bit rocky for J.R. Giddens. Being on a championship team won't make his quest for playing time any easier.

The Boston Celtics first-round pick out of New Mexico is learning how much he has to learn.

"I'm not going to lie," Giddens said after Friday's practice. "I'm a little frustrated right now because it's hard to pick up on things and I'm not picking up on them as fast as I would like. But that's part of being a rookie.

"The older guys tell me that's how it is as a young guy. You've got to come here and stay positive."

The 6-foot-5, 215-pound Giddens played two years at Kansas before transferring to New Mexico. As a senior, he ranked fourth in the Mountain West Conference with 16.3 points per game, first in rebounding with 8.8 per game and fourth in field goal percentage at .516. He's the only guard who has led the Mountain West Conference in rebounding.

The Celtics chose him with the final pick of the first round, the 31st overall, and like his athleticism and quickness.

But young players like Giddens don't realize that the transition to the NBA takes time, coach Doc Rivers said.

So Giddens "is behind because he doesn't know any of our" plays, Rivers said. "All the young guys are (behind) because of that. He's getting better each day but he has a long way to go.

"The problem usually with young guys is coaches have patience and they don't. He wants it now so he gets frustrated. It's going to take a while, but I'm not in a hurry so it's not a big deal for me."

Giddens joins a team that went 24-58 in 2006-07 then won its 17th NBA title last season after adding Kevin Garnett and Ray Allen in trades and James Posey and Eddie House through free agency. Posey became a free agent and signed with New Orleans after the season.

Giddens is trying to fit in.

"I've got to learn a lot of things, like knowing when to foul and when not to foul, and knowing how physical I can be with a shooter and what refs call fouls," Giddens said. "I'm so used to college rules that I don't know how physical I can be with some of these guys.

"It's an adjustment."

He is excited about practicing with Garnett, Allen and Paul Pierce, all NBA stars.

"After seeing them on TV, seeing them in person is a lot better," Giddens said. "These guys are great defensively and are great leaders. I think each brings a different key to the team which makes them even better.

"KG is a monster. He's the heart of the team and brings it every day. Ray is probably the most mild-mannered of them all and is a very sensible guy. Paul is vicious. He's like a video game in person."