The San Antonio Spurs are sitting on top of the league with a 40-12 record. They have done this through a combination of offense and defense, ranking in the elite in most of the regular categories that are used to rank possible contenders - points per game, points per possession, offensive rating, field goal percentage, and three point percentage; not to mention opponent points per game, opponent points per possession, defensive rating, opponent field goal percentage, and opponent three point percentage. Nevertheless, if they have a weakness apart from their turnover woes (which I'll admit have been improving lately), it is their consistent inability to garner offensive rebounds.
The Spurs have a dismal 20.4 offensive rebounding percentage, which means they grab a mere fifth of their missed baskets. That places them dead last in the Association, even behind traditionally weak offensive rebounding teams like Boston (who now place 29th) or Miami (who place 27th). Now, what could be the reason for this weakness in the Spurs' vaunted offense?
Preventing transition baskets
The first thing to consider is that the Spurs have never been a team to chase their own misses because they're focused on preventing their opponent from getting easy baskets. However, the Spurs give up nearly 14 fast break points a game, 24th in the league, and around 169 points per 100 fast break possessions, which is 14th in the league. By contrast, Indiana, 5th in the league in offensive rebounding, leads the league in opponent fast break points allowed and opponent fast break efficiency. So ignoring the offensive glass to get back on defense doesn't seem to be the answer to our questions.
Pop wants the team to push the pace
This may be one of the reasons. The Spurs currently average 94 and a half possessions in a game, good for fourth in the league. The thing is that offensive rebounds are counted as continuations of possessions rather than a new possession in their own right. This is in line with the Spurs' evolution into a team that focuses on an uptempo, guard-driven offense, rather than a post-oriented, slow-paced offense. Nevertheless, the Spurs are having one of the worst offensive rebounding seasons in history, not even the Seven Seconds or Less Suns were this allergic to second chance points.
The Spurs need to be more athletic
One of the features of Pop's system is that it compensates for something the Spurs cannot easily acquire: athletic talent. With the Big 3 aging and the Spurs not being in a position to snag top draft picks, where the top athletic prospects are usually drafted, the Spurs have had to scour the bargain bin for less athletic but still valuable contributors, like Danny Green, Boris Diaw, and Matt Bonner. The Spurs' most athletic player, Kawhi Leonard, was drafted 15th and the Spurs had to trade George Hill in order to move up that high.
Offensive rebounding is partially a function of athleticism, although it also helps to be great at positioning (hint: being tall AND fat helps, just look at Zach Randolph). So maybe the Spurs just need a tall and stout player in the pivot.
The Spurs' offensive (and defensive!) system means there's no time to grab offensive boards
The Spurs offensive scheme features a lot of motion by all five players on the court. The lead guard alternates between handling the ball and moving off-ball while a big has it, the wings scurry around and try to get free on the wings, at the corner, or under the basket, and the bigs set screens over and over again. Understandably, a motion offense where even the biggest players are setting screens near the top of the key midway through the shot clock means that they have fewer opportunities to grab a loose ball.
For that matter, the Spurs team defense is all about denying easy baskets, particularly from the (corner) 3, the paint, and the foul line. Trying to volley for loose balls risks a Spur drawing a loose ball foul, which can put the other team in the bonus relatively quickly. The Spurs have remained elite in preventing opponents from getting to the line and converting, as their top 4 mark in FT/FGA allowed shows - and this is after they placed 2nd in this ranking last season.
Do the Spurs need to grab their own misses to win it all?
Now let's take a look at the offensive rebounding percentages of the 5 previous champions:
2012 Miami Heat: 26.6 (19th)
2011 Dallas Mavericks: 24.1 (26th)
2010 Los Angeles Lakers: 27.6 (7th)
2009 Los Angeles Lakers: 29.4 (3rd)
2008 Boston Celtics: 26.6 (18th)
So while it's unsurprising that the Lakers won rings through being much bigger than their opponent (and having a no-conscience gunner, hence the notion of the Kobe assist) it turns out that you don't have to be too focused on grabbing your own misses.
The bottom line is that you have to make shots while preventing your opponent from making theirs. And the Spurs are passing this test with flying colors, though they're definitely missing Tim Duncan and Manu Ginobili in that regard. So while the Spurs definitely need to get well, getting healthy on the offensive boards doesn't seem to be a requirement to reach their ultimate goal: that elusive fifth ring.
Stats care of Basketball Reference and Team Rankings.