“I told him, No. 1, I shouldn’t have played him, in my opinion,’’ Rivers said of Pierce. “He talked me into it and I shouldn’t have played him. No. 2, he’s not going to practice [yesterday or today]. I told him that after the game.’’

Pierce, though, did participate in agility drills yesterday, then went to the sideline as the Celtics scrimmaged, working out on a stationary bicycle, then going to the locker room to have his knee taped. Pierce’s status for tomorrow’s game against Golden State at TD Garden has not been determined.

“He actually tried to get on the floor at the beginning and we told him to get off,’’ Rivers said of the workout. “He doesn’t miss practices. The only way you can get him to miss, it has to be a force action.’’

Pierce sustained the injury late in the third quarter vs. Atlanta Friday night, then had 15 points in 34 minutes at Indiana.

Pierce left the game against the Hawks with 3:16 remaining in the third, returning to the bench wearing a knee brace, then played the final 10:41 of that contest. In 44 minutes since the injury, Pierce has 21 points on 6-for-15 shooting and is 8 for 12 from the line.

“I still think the amount of games does have an impact,’’ Rivers said of the Celtics’ struggles. “I thought I made a mistake on Thursday, I thought we went way too hard in practice, and I thought it carried over to our back-to-backs on Friday and Saturday. I knew that and I thought we could get away with it, and I was wrong. So that’s some of it. The other part is - a lot of it is technique, just getting the technique down, but that always goes back to the focus issue.’’

The Celtics (8-3) have lost three of five games after starting the season with six wins.

“I thought, obviously, through training camp and the first few games, we got off to a great defensive start,’’ Rivers said. “And I think we thought it was going to be easy from that point on, and it hasn’t been. So we’ve got to take that next step - and we will. Pick-and-roll coverage has been inconsistent and it’s not just two guys. It’s not just the guy on the ball, it’s the guy guarding the guy setting the pick.

“Some of our defensive stands have been very good and then, five seconds on the clock, and the guy beats you off the dribble and gets all the way to the basket. You’ve got to guard the ball better, then there’s got to be a secondary guy, then there’s got to be a third guy. That’s why you watch film and have practices. We worked on pick and roll and we’ll work on it a lot [today]. Transition and pick-and-roll D, then the rest will follow.

“Like I told them after one of our losses, I would love to tell you that what they were doing was beating our stuff - but we were never running our stuff. We didn’t execute our stuff, defensively.’’

Rivers was asked if athletic teams give his squad trouble.

“Yeah, [they] do bother us, but we’re going to play them and we’ll be fine with that,’’ he said. “The opponent is us every night and that’s going to be a challenge. I think we win a couple games and we tend so far not to have the 48-minute capacity yet.

“That’s not anything that’s alarming but I know to be great we have to be a 48-minute team. Right now, we’re just not. We go in and out, so that’s just something we’ve got to improve on.’’

“I wouldn’t call it slippage,’’ he said. “It’s kind of weird, because one thing leads to another, which leads to another thing. Slippage is when you stop being disciplined, when you stop being consistent with what you started with, and I don’t think we’re that. You give guys free rein or confidence, anybody can score in this league.

“We got outrebounded both games. You can’t win like that. Are we as crisp as we could be? No. I think that’s what I’ve always been saying. We are not that team, as polished as we can be. We worked on some things, polished some things, we’re getting back to our principles.

“I’m not going to hit the panic button. I’m pretty sure everybody is capable of looking at themselves and saying I could do this better or I could do that better. We’re trying to come out with a lot of energy. But it has to be a defensive effort, a defensive focus. I think for the last two games we haven’t been as crisp as we want. We have to respect everybody on the floor.’’

Garnett said something can be learned from every loss.

“You can figure out something about yourself and your team,’’ he said. “And the more you figure out, the better.

“You don’t always want to figure it out with a loss [but] everything that is perceived as a negative does have some good points in it. It does make you hungry. We’re a very competitive team. We’re a prideful team. And this does put some fire under our feet a little bit.

“No one likes to lose, including everybody in here. We take a lot of pride in our defense and how we come out and approach the game. We never said we had a perfect defense. We have flaws, like anybody else, and we’re just working on those flaws.’’

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