Blair's rookie season, he averaged 10.6 defensive rebounds per 48 minutes. Last year it dropped to 9.5. This year, even after tonight, he's sitting at 6.0.
Offensive rebounds are nice but the Spurs are built on cleaning up the defensive glass. Blair becomes much less valuable if he goes from elite defensive rebounder to an average or below average defensive rebounder.
I think it's probably a case of small sample size but it's definitely something to keep an eye on. Usually the first sign of a small power forward losing their athleticism is a drop in defensive rebounding rate. And with Blair's knee situation, no one really knows when that drop will come. We could be witnessing the drop already or he could have a complete career.