By Cody Westerlund-

(CBS) Considering the results of the past four years under coach Tom Thibodeau and the solid two-way play of big man Pau Gasol, the most puzzling aspect of the Bulls on the court this season has been the regression of their defense.

It’s been un-Chicago-like in that it’s been merely … average.

After ranking in the top three in scoring defense each of the past four years and in the top two of field-goal percentage defense three times in that stretch, the Bulls ranked 20th in scoring defense (100.5 points per game) while ranking a respectable-but-not-special eighth in field-goal percentage defense (.440) through Monday night.

The advanced metrics that adjust for pace reflect a little better on the Bulls, as their defensive rating of 102.6 is 10th in the league, according to NBA.com – but nowhere near the league’s elite, which most have come to expect from Thibodeau-coached squads.

However you shape it, the Bulls have a rather average defense nearly one-quarter of the way into the season, a fact they readily admit.

So why is this? There are a couple main factors.

Significantly, injuries have hampered Chicago, with the likes of Gasol, Joakim Noah, Taj Gibson, Jimmy Butler, Derrick Rose and Kirk Hinrich missing multiple games. A cautious approach mandated by the front office and training staff has led to restrictions as well. This has severely limited how often the starting five has practiced together – “very, very, very few times” times together, Thibodeau said Saturday night.

That health challenge has contributed to a second main issue, the one the Bulls can control – trust on defense. That’s been magnified as Noah has largely moved from playing center on defense to power forward and new pieces, including Rose, have worked their way in.

“We’re just not trusting right now, not having each other’s backs on certain things,” Gibson said Saturday. “It’s a small thing that you can fix, but you got to get it right.”

Because the Bulls will likely have to go through LeBron James, Kevin Love and the Cavaliers if they want to win the East, it’s worth examining how Chicago’s fared against opponents who start stretch fours – power forwards who regularly spread defenses out by stepping out to the 3-point line and being a threat.

We got our best look last week when Chicago hosted Dallas and future Hall of Famer Dirk Nowitzki, followed by a visit to town from Golden State and Draymond Green. The Bulls struggled defensively and lost both games, in part because both foes made a point to stretch the floor using their four men, who either executed (in Green’s case) or opened up other opportunities (in Nowitzki’s case).

Dallas made 15 3-pointers against Chicago, and while Nowitzki only had two, he was directly involved in seven treys converted by teammates – whether it be in the screen-and-roll or his ability to draw defense and be a threat from anywhere.

Sometimes, it seems like Noah’s old habits can get the best of him – he’s drawn to the middle of the lane in the clip below, even when he’s not needed there.

As the Bulls chose to not let Warriors guards Steph Curry and Klay Thompson beat them, Golden State responded by putting Noah – who’s been hobbled by ankle and knee injuries – through the ringer time and again with Green in screen-and-rolls. Green had a career night in hitting 7-of-13 3-pointers.

With Noah having to guard more players out to the perimeter, his lingering knee and ankle issues are a serious concern. He’s taking longer to close out on shooters and recover than when he won the Defensive Player of the Year award in 2013-‘14, and it’s shown, in unassuming situations and otherwise.

In total, Chicago has played six games this season against teams that start a stretch-fours (for purposes of this piece, that’s defined as a power forward who’s making at least one 3-pointer per game, which is the threshold for qualifying for the 3-point percentage crown). The Bulls are 2-4 in those games, which also includes two contests against the Celtics (believe it or not, Jared Sullinger is a consistent 3-point threat this year), one against the Cavaliers (Love) and one against the Magic (Channing Frye).

In those outings, the opposing starting stretch fours have combined to shoot nearly 44 percent from 3-point range. They’re averaging 18.5 points, which isn’t crazy – but the Bulls are giving up 109.3 points to those teams. As the Warriors and Mavericks prove, it’s natural that a lot of teams with four men who can stretch the floor would be better at offense, because it creates better floor spacing.

That said, stretch four men also may be the kryptonite to a staple of Thibodeau’s defense. One of the Bulls’ principles is to “ice” potential ball screens on the wing – in simple terms, don’t let them happen.

At its most basic, icing is a defensive tactic in which the on-ball defender plays higher up on the ball-handler and wedges his way into the path toward the would-be screener – to block the middle of the floor on a side screen-and-roll. This gives the ball-handler some room to maneuver away from the potential pick and toward the hoop, but the idea is for Chicago’s big man defender involved in the action to corral the ball-handler until the primary defender (usually a guard) can recover and get in front.

Icing works well when the would-be screener is a big man who doesn’t shoot well, for any pass back to them isn’t threatening. Stretch-four men make this principle far more complicated, because you can’t leave them to hang out by themselves, as this Green 3-pointer proves.

That clip shows quality action from the Warriors away from the ball, which is why the Bulls weren’t in position to rotate properly. Still, the mere presence of Green causes difficult decisions here and shows why the entire defense must pitch in.

Another one of Thibodeau’s defensive tactics in pick-and-roll situations (including those in the middle of the floor) is that he wants one of the two offensive players involved to shoot, as Grantland’s Zach Lowe has previously detailed well, so that the rest of the offensive players don’t get good looks.

“That philosophy is based on a rather bold belief: The Bulls think their two defenders, with just a little bit of help, can beat your two offensive players and coax the exact kind of low-efficiency shot you don’t want to take,” Lowe has written.

When a capable big man pops behind the 3-point line on a screen-and-roll that goes does come to fruition, that long 2-pointer suddenly becomes a high-efficiency shot, to Thibodeau’s chagrin.

Only 20.8 percent of Bulls’ opponents’ shots this season have been 3-pointers, second-fewest in the league, as Thibodeau prefers. In those six games against stretch fours, that number is 23.4 – still solid, but opponents have capitalized on their chances.

In short, stretch-four men force Chicago out of some primary tendencies it uses. So what’s the solution in those situations for the Bulls?

Time (and health) would certainly help, as the players need to become more familiar with each other — to get familiar with the needed tendencies. And Thibodeau’s a defensive savant, so you figure he’ll come up with something that works in the next four months.

Perhaps part of it needs to be personnel-drive as well. By far, the Bulls’ best big man defensive pairing is Gasol and Gibson together — their defensive rating is 92.4 this season, according to NBA.com (Gasol-Noah’s is 101.7 and Noah-Gibson’s is 102.2). That jives with the eye test, as Gibson is more capable right now than Noah in guarding to the perimeter, while Gasol has done well in protecting the rim.

Interestingly, the defensive rating of Noah paired with rookie Nikola Mirotic is a solid 97.7. Perhaps there’s noise in that coming against more second units, but it reinforces that Noah is more comfortable playing center.

As we can begin to draw conclusions with 20 games played, Chicago’s defense is under the microscope in a new manner with Cleveland likely waiting come playoff time.

It will have to get fixed, or the Bulls won’t accomplish their ultimate goal. And they know that.

“What … we want to do – and that’s go ahead and play for a championship – we can’t let these things happen,” Gibson said. “You can’t win too many games being lackadaisical on defense.”

Cody Westerlund is a sports editor for CBSChicago.com and covers the Bulls. Follow him on Twitter @CodyWesterlund.