PHILADELPHIA -- What was the problem with the Bulls defense on Friday night?

Allow Tom Thibodeau to list the ways.

"From A to Z," said the frustrated coach after the game.

Thibodeau's starters didn't exactly bring the passion and fire to the defensive end that they appeared to have on the offensive side. After struggling to lock down the lowly New Jersey Nets on Wednesday night, the same issue reared its ugly head in a 105-99 loss to Philadelphia 76ers on Friday. Their rotations weren't crisp, and yet again they looked lethargic from the outset.

"Our catch-and-shoot defense wasn't very good," Thibodeau said. "Containing dribble penetration. And then challenging shots, you got to challenge shots. Sometimes we're right there and if we don't challenge shots they're going to make [them]. They got in rhythm early on us and when a team gets confidence, it's much harder to slow them down. They got their confidence going and we had a hard time all night."

The lagging defense is a trend that Thibodeau and his team are sick of talking about, but they don't seem focused enough to turn things around. Since Joakim Noah left the lineup on Dec. 18 because of thumb surgery, the Bulls have struggled to find their defensive swagger against bad teams. Over the past two games, the Bulls haven't been able to get their defensive energy up to a level where they can dig themselves out of the hole that they've created.

"When you're not playing defense, it's going to hurt you," Bulls point guard Derrick Rose said. "And we didn't do it tonight."

It’s worth noting that the 76ers shot the heck out of the ball. For a while, it seemed like they couldn't miss. They shot 56 percent from the field. But it wasn't like the Bulls had poor shooting night, either. They shot 51 percent and routinely got the looks that they were searching for.

The Bulls' malaise of the past few games was typified during the third quarter Friday night. The Nets shot 78 percent from the field during that 12-minute stretch and simply toyed with some of the Bulls' sets.

"We weren't playing any defense," Rose said. "I think they hit 10 or 11 shots in a row. We couldn't stop nobody. But we can't panic. We just lost two in a row. We got a game [Saturday]."

While Rose talked about not panicking, he also said that he isn't sure how the Bulls can fix what ails them at the moment. Noah isn't coming back for at least another six weeks and the Bulls appear to have lost the one constant they've carried with them throughout most of the season: Defensive intensity.

"I really don't know right now," Rose said. "I'm confused. We just got to find a way where everybody's on the same page ... We're not playing like ourselves right now. Where we're not stopping people, they're scoring at will right now and we just got to get back to ourselves."

Carlos Boozer was pretty much the only silver lining for the Bulls on this night, responding with 31 points and 13 rebounds after being benched Wednesday night in New Jersey. He had another, simpler idea as to how his team can get back on track.

"We can't let teams shoot over 50 percent on their home court, or any court, and expect to win those games," he said. "We have to do a better job on D. On offense isn't really the problem ... we just got to do a better job -- point blank. And we got to do it overnight because we play a good team [Saturday]."

There's no doubt about that, considering the Boston Celtics are coming to town. If the Bulls continue to play defense the way they have against the Nets and Sixers, the frustration Thibodeau expressed over the last few days is going to turn into something completely different ... rage.