I didn't want to go look it up, but that's about what I would have guessed. That probably says more than just looking at raw RPG. They could do better than 11th for R%, but that still shows that some of the drop in RPG is due to fewer opportunities.
I just had a look at teamranking.com - the Spurs defensive R% is 76.2%. That's 6th in the league. Maybe they could do better, but it's not too shabby. For last year (the complete season) their defensive R% was 76.0.
The Spurs offensive R% is 24.4%, compared to last year where they got 26.5%. They are ranked 21 in offensive R%. That's not good enough, and they need some improvement. BUT... what a lot of Spurs fans don't want to accept is that the Spurs are always near the bottom in offensive rebounds. That's because Pop's philosophy has always been to get everybody back down the court on defense, rather than focusing on the offensive glass.
That makes a lot of sense, when you are holding your opponents to a low shooting percentage defensively. It's a bad combination to give up the offensive glass, AND to give up a higer shooting percentage. Maybe the problem is somewhere in the middle. They may be fighting for a few more offensive boards (Blair), and not getting them. And that would also keep them from getting set on defense as well as they have in the past, which gives the opponents a higher shooting %.
Either way, they appear to be about as efficient on the defensive boards as they were last year. It's the offensive end they are falling down on. That, and letting opponents shoot 48%.