The Celtics went through a longer than expected practice Saturday, with Doc Rivers placing special emphasis on the team’s inability to contain dribble penetration.

“We worked on a lot of defensive stuff because the dribble penetration is just crushing us,” Rivers said. “We showed film of the last three games, and you don’t have to talk a lot. I mean, it’s pretty obvious. Guys have the ball square and then the guy’s driving. You see the big (the forwards or center) running over to help and either it’s a foul, it’s a layup or it’s an offensive rebound.

“I don’t think that’s that hard to figure out. We just have to do a better job on it. That should never happen, especially on some of the plays when it’s happening. There’s no reason for you to get beat off the dribble.

“And then,” Rivers added, “our bigs have to do a better job of talking, because a lot of times, in the guards’ defense, they’re thinking that there’s a pick and roll coming — there’s nothing, but they don’t know. They’re guarding the ball.”