The skidding Nets have lost four of their last five and fallen to sixth in the Eastern Conference. Deron Williams criticized their energy and effort on defense, and interim coach P.J. Carlesimo admitted fixing the defense and turnovers are their most important tasks — important enough he’s willing to shuffle the lineup to do it. “We’re definitely slipping, so it’s definitely a cause for concern, and we want to work our way back toward the top,’’ Williams said. In their five-game funk, his Nets have coughed up 94.6 points on 49.2 percent shooting, and 43.7 percent from 3-point range. The former is sixth-worst in the NBA and the latter third-worst. With defense like that, it’s no wonder the Nets — who face the Bobcats in Charlotte tomorrow — have slid five games in the loss column behind the Atlantic-leading Knicks. “We have to bring better effort, better energy, better defensive presence,” Williams said. “Watching film, the last couple games we definitely see a slip in our defense, and we can play a lot better. It’s on everybody. You can talk about it all you want, and we’ve done that. It’s coming out and doing it.’’ Williams wasn’t excusing himself. He holds himself accountable, especially for the Nets averaging an uncharacteristic 17.4 turnovers in their recent malaise. “A lot of them are just me being careless with the ball,’’ Williams said. “I’m not going to blame it on anyone but myself, because I’m the one who has the ball.’’ Carlesimo acknowledged a need to correct both the defense and the turnovers. But the former he hopes to fix with some lineup changes, and the latter he hoped would be helped by Joe Johnson’s return to health. “We need to defend better. We need to not turn the ball,’’ Carlesimo said. “But it’d be foolish to not say [that] we played three games without Joe and two games with Joe not 100 percent yet — and some of them against pretty good teams — and not think that would be a factor.’’