“You have to be in shape first,’’ Rivers said. “You can have all the intensity you want, and I think our guys do. But after three minutes and your lungs hurt, you’re no longer thinking about intensity, you’re thinking about survival. How can I get through this next five minutes? And then you’re not playing the same. So, we have to be in better shape.’’

Rivers subjected the Celtics to a lengthy review of Friday’s 87-74 loss to the Pacers, then put them through a two-hour practice.

“Some of it is we have to be tougher,’’ Rivers said. “Maybe that’s conditioning, as well.

“Both teams played pretty awful offensively. Indiana was tougher. The game could have been won by either team. Nobody wanted it.

“We watched a lot of film and it showed. I showed our guys we had five times where we had inside position on a rebound, our ‘bigs’, and they still got the rebound. To me, that’s physicality, that’s not boxing out, that’s relying on your athleticism. Five times, and they came up and scored on every single possession. That’s the game - the other night, that’s the game. So, obviously, we have to do a better job.’’

Rivers, however, praised the Celtics’ camaraderie.

“We can get along but I want to win, too,’’ Rivers said. “Chemistry is phenomenal. I couldn’t ask for a better group of guys but I may be asking for a tougher group of guys.

“Right now, we’re not a post team. We haven’t proven we can be that. We’re a jump-shooting team and a pick-and-roll team. When you see jump shots, it’s probably conditioning.’’

Yesterday’s workout was “sloppy, but it was good,’’ Rivers said. “It was long, a ton of running, that was done with a purpose. We need this.’’

Rivers said the hurried approach to preparing for the season has contributed to the Celtics’ problems.

“In practices we should have done more running and not worrying about the results of the game,’’ Rivers said. “That’s where we’re kind of learning on the fly here. I learned watching us play the last couple games. We’re tired after three minutes, four minutes. So, that’s a lesson learned.’’

The uncertainty about the start of the season has also been a factor.

“I think this year, I guarantee half the guys, when they decertified, thought ‘no season’ or ‘January, at the earliest,’ said Rivers. All of a sudden, it came together within a week, and it was too late to get in shape. That probably threw a lot of guys off.’’

Celtics captain Paul Pierce, who has recovered from a bruised heel but missed most of training camp, has had even less time to prepare than most of his teammates.

“You forget he missed everything,’’ Rivers said. “We did a new drill in training camp and Paul messed up three times. We forgot he didn’t know the drill because he didn’t have a practice.’’

Green surgery a success Jeff Green underwent successful surgery to repair an aortic aneurysm yesterday, according to Rivers.

“Surgery went well, they’re saying he did great,’’ Rivers said. “And, obviously, our thoughts and prayers, and every part of our being is with Jeff right now. The second thing we want to hear is he will play again. That would be terrific and I don’t care if it’s with us. I want it to be with us. I just want him to fulfill his life, at the end of the day, that’s what we want.’’

Ready or not? Mickael Pietrus could make his first appearance for the Celtics (4-4) when they host the Mavericks tomorrow. Pietrus, who was signed Christmas Eve, has been strengthening his surgically-repaired right knee.

“Obviously not playing, his timing [needs work], but I thought as practice went on he got better and better,’’ Rivers said. “I just think he’s starting to get his legs.

“I haven’t even asked [about tomorrow’s availability], I’m not sure. I’ll find that out [today]. He’s another piece that can help us. You can see his toughness, his defense. He made three or four 3s that were great to see. So that adds to the second unit.’’

Pietrus said he plans to be available for the Dallas game.

“I went through my first practice and it’s good to be a Celtic,’’ Pietrus said. “I think I’ll be ready [tomorrow]. No pain, and I said I would be ready in two weeks.

“It feels good. I mean, look around, so many banners - so I’m in a great spot and just try to work hard and be ready to go.

“I haven’t played for eight months but, defensively, you can always help your team to do those things to be a great team. Hopefully, if I get in the game for five or 10 minutes I can help my team.

“It’s always exciting, it’s why I always wanted to be a Celtic - [Kevin Garnett], Boston, that toughness when we played them. They were so tough it was hard for us to get a shot. I’m trying to get the same mind-set. So, get in the game, play hard and make sure we work hard and at the end of the day we get a ‘W.’ ’’

Frank Dell’Apa can be reached at f_dellapa@globe.com.