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The health of a certain All-Star's left knee could very well be the linchpin to the Chicago Bulls' hopes of reaching the NBA Finals for the first time since Michael Jordan hung that goose-necked wrist nearly 17 years ago in Salt Lake City.

It's just not the one belonging to Derrick Rose.

If there's a reason to worry about the Bulls' wobbly first half of the season, it has to do with the slippage in their defense. If there's one player responsible for that, it's Joakim Noah, the supremely versatile and tireless big man and reigning Defensive Player of the Year who has served as the backbone to coach Tom Thibodeau's suffocating system for the last several seasons.

This is the same Noah who had surgery on his left knee last May after helping drag the Bulls to a playoff berth despite Rose's absence. The same surgery the team referred to as "minor" but required eight to 12 weeks of recovery, twice the normal time of a standard arthroscopic procedure to clean out the joint or deal with a small tear.

The same Noah whose shooting percentages are at career lows and whose rebounding and scoring haven't been this scant since he established himself as a starter six seasons ago.

Noah isn't the only reason, of course, for the Bulls' marshmallowy "D," which has slipped from 97.8 last season to 102.0 this season in terms of efficiency.

Shooting guard Jimmy Butler admitted that the Bulls' new arsenal of offensive weapons has softened the team's collective conviction to stop opponents. "That's the reason and it has to change," he said. "Right now, we just think we can outscore teams. That's not going to win no championship. We talked about it. If we can get back to the way we were defending, holding guys to 85, we'll get it."

In previous seasons, though, the conversation wouldn't have happened only in practice. Noah would've inspired more defensive intensity, by both word and action, before it resulted in multiple defeats by big margins.

After Monday's unsettling home loss to the Orlando Magic, who shot nearly 60 percent for the game and scored an opponent season-high 121 points in regulation, Noah rerouted questions about his knee better than he has the opposition this season.

He shook his head at any suggestion that there are times the knee is troubling him. Only when a reporter asked if it has improved steadily did he say, "Yes, yes, yes."

Charles Rex Arbogast/Associated Press

League sources contend, however, that Noah has privately expressed concern about the knee. They point out that he is not nearly as active or talkative organizing the Bulls on defense, signs that he's preoccupied with "managing the pain," one source said.

He practiced Tuesday with extra neoprene wrap and tape around it and worked afterward against Nazr Mohammed on sweeping into the lane for left-handed jump hooks, which allow him to jump off his right leg.

Without referring to Noah directly, Rose did acknowledge that the lack of both communication and energy are the reasons the team's defense hasn't been the same. "Communication and energy are a big part of our defense," he said. "We're not focusing. The concentration isn't there."

The Bulls also are incorporating a new big man, Pau Gasol, into the mix, and while he has certainly done wonders for their offense and is boasting impressive rebounding and shot-blocking statistics, watching him play against the Magic told another story.

A defender can make the effort to contest a shot without expending—or having—the energy to truly bother an opponent or alter his shot.

Magic center Nikola Vucevic backed down Gasol repeatedly for 33 points on 16-of-24 shooting, including a vicious dunk that sent Gasol sprawling. Magic guard Victor Oladipo, meanwhile, pulled up for eight-foot floaters over Gasol's outstretched arms to provide another 33.

Gasol has plenty of reason to be a step slow. He's 34, in his 14th season and, combined with international competition for Spain, has played as many minutes over the last decade as any big man in the league.

His comfort playing closer to the basket has also forced Noah to defend stretch 4s more routinely, which only means more testing of that left knee's sturdiness.

Overall, advanced statistics indicate no one is better at defending shots within three feet of the basket than the Bulls, with opponents shooting a league-low 58.5 percent. But they've also allowed the fourth-most layups, which suggests opponents aren't afraid to attack the rim either.

Although the United Center turned nervously silent Monday when Rose was tripped driving to the basket, landed hard and was slow to get up, he offered glimpses in both halves that his breathtaking explosiveness is intact despite playing only 10 games over the previous two seasons.

In the first half, he accelerated out of the backcourt at a speed that turned the floor into a set from The Matrix, every person and object appearing to freeze as Rose glided past them.

In the second half, he made a similar burst with Channing Frye between him and the basket. Frye had no choice but to simply lunge and grab him for a foul.

"Oh, he's still got it," said Magic backup point guard Luke Ridnour. "He looks like he's all the way back to me."

If there was any question that Rose can be the Rose of old, he answered it with his 32 points against the Wizards on Wednesday night, a game in which Noah sprained his right ankle and played only 13 minutes while the Bulls gave up 100-plus points for the 19th time this season.

That's one more triple-digit allowance than they gave up all last season, playoffs included.

Rose, to be fair, has been wildly inconsistent, but is that really all that surprising for someone who has missed most of the previous two seasons? The criticism of his three-point shooting is understandable but shortsighted. He was 6-of-9 from three vs. the Wizards but is shooting 27.8 percent overall.

Sources around the team insist he isn't forcing threes or taking them out of rhythm.

They also have little to do with the Bulls' defensive issues, seeing as their transition points allowed are essentially the same as last year's.

Big picture: For the Bulls to give Gasol space inside and Rose not to wear himself down constantly attacking the rim when it matters most—the postseason—he has to develop into a long-range threat. Not shooting threes now when appropriate isn't going to make that happen.

Charles Rex Arbogast/Associated Press/Associated Press

Asked if last year's Herculean effort to make the playoffs without Rose—who was out with torn cartilage in his right knee after the previous year was spent recovering from torn ligaments in his left knee—sapped the Bulls' energy to defend, Butler vigorously shook his head.

"It's definitely still there," Butler said. "Defense is all will. We just have to man up and guard our guy. There's only so much strategy to defense. Play hard, stop your man, put your body on the line. We have that in us. If we want a ring, one of these banners, we have to guard."

There's never been a reason to question Noah's desire or willingness to play hard, guard or put his body on the line. Or to take responsibility for what needs to improve.

"I feel as if we still have one of the better frontcourts in the East," Noah said. "We just have to find ways to be more effective."

With the season halfway over, it appears the Bulls are still looking.

Advanced statistics courtesy of ESPN.com and Basketball-Reference.com.

Ric Bucher covers the NBA for Bleacher Report. Follow him on Twitter @RicBucher.