[Note: As indicated, this was written before the Bucks win on Wednesday. But even then the Bulls gave up a 34% offensive rebound rate -yfbb]
Through 4 games, Tom Thibodeau's defense is outside of the league's top 10 in points allowed per 100 possessions and that is, obviously, very surprising. Granted, Chicago is 11th in defensive efficiency, so they are barely outside of the top 10, but given that under Thibodeau the Bulls have finished 1st, 1st, 5th, and 2nd in overall defense in the last 4 seasons, it is a bit surprising to see them anywhere but the very tip top of the league's defensive rankings, even at this early juncture.
Upon examination of the Bulls league-wide rankings in the defensive four factors, it's clear that defensive rebounding, an area that used to be a team defensive strength, has, at least after 4 games, become a big weakness. As a team, the Bulls are surrounding an opponent offensive rebounding rate of 31.1%, 3rd worst in the league, ahead of only the Dallas Mavericks and Golden State Warriors.
The Bulls entire team has been pretty bad at snagging defensive rebounds and keeping their opponents off the offensive glass. This was most glaring in the Bulls lone loss of the season against LeBron's Cleveland Cavaliers, as Tristan Thompson snagged offensive rebound after offensive rebound, as the Bulls just could not finish their defense by securing the basketball.
Looking at the following graph, which shows the Bulls Opponent OREB% with each of its players on and then off the floor and the resulting net OppOREB%, you can see that the Bulls keep opponents to the lowest OReb% when Joakim Noah is on the floor, at 27.4%. That seems pretty good for Joakim, until you realize that a team surrendering offensive rebounds to their opponent at that rate would rank roughly 22nd overall in the league thus far, and that's the Bulls at their best this season.
Another thing to take note of from the chart above is that the Bulls starters seem to be doing much, much better than the bench of keeping opponents off the offensive glass, even though the starters haven't been so great at it. From a personnel perspective it's not surprising that the Bulls' bench unit isn't great at defensive rebounding and keeping their opponents from getting extra possessions on the glass.
Aaron Brooks is tiny and Doug McDermott was never much of a rebounder in college. Tony Snell is rail thin and is always going to struggle to box his man out. Nikola Mirotic is awesome, but he was never a great defensive rebounder in Europe by any stretch. Mirotic does sport a relatively gaudy, team-leading individual DReb% of 28.8%, but looking at how a team rebounds when a player is on the floor is often a much better measure of the quality of the rebounder, as it better accounts for boxing out and doesn't credit guys for grabbing uncontested rebounds. Looking at the on-off splits for opponent rebound percentage, Nikola's strong DReb% seems like a bit of a mirage, thus far. On the other hand, with such a small sample of minutes to look at, it's also possible Niko's rebounding looks worse on the on-off splits because he spends so much of his time playing with guys like Brooks, McDermott, and even Taj Gibson, whose defensive rebounding has fallen off a cliff this season so far. Taj's hitting the defensive glass like a guard so far in the season grabbing just 7.7% percent of available defensive rebounds when he's been on the court, compared to a more robust 16.5% last season, much more in line with his career numbers.
It's easy to look at things and totally blame the bench, but as I mentioned earlier, the Bulls haven't exactly been dominating the defensive glass with the starters on the floor either. No one has been very good. It's possible that the Bulls actually miss Carlos Boozer, who, despite all of his problems elsewhere on defense, was always a tremendous defensive rebounder. Despite some claims to the contrary, Boozer's defensive rebounding was not just the uncontested variety where he padded his stats. According to the GotBuckets.com's Four Factors Adjusted Plus Minus, Boozer was consistently very good at raising the Bulls' team defensive rebounding percentage when he was on the floor. In his time in Chicago, he was never lower than the 83rd and as high as the 95th percentile in the entire league at raising his team's defensive rebounding percentage, adjusting for his teammates and opponents. That is to say, he wasn't just leaching rebounds from his teammates.
In fairness to this year's Bulls, these results are likely pretty skewed by the Tristan Thompson game, and a four game sample is pretty hard to get worked up over (well, unless you're me). In addition, the Bulls are incorporating a lot of new players and they are dealing with a bunch of injuries which have prevented much lineup continuity in the early-going of the season. Boxing out and actually rebounding, when done well, is usually a team exercise and with better communication and understanding, the Bulls should improve on the defensive glass.
The final thing that's important to note is that Bulls are still as good as ever at forcing misses. They currently sit second in the league in opponent eFG%, behind only Golden State. Given that eFG% is by far the most important of the four factors and the likelihood that the Bulls progress towards the mean of their normal performance on the defensive glass, they should return to their elite defensive status in relatively short order. Either way, it's something I'll be keeping an eye on going forward.
Stats for this piece, unless otherwise cited, via NBA.com/stats.