Considering how 2 of our starting 5 were backups last year, and 2 others played behind Millsap and Josh Howard just a few years ago, under the worst coach in the league, I think they're doing just fine.
This is why fans were clamoring for the kids 3+ years ago over the vets. We could have taught them how to play defense via actually playing. We could have assessed who really cared enough to play defense and who had the attitude. Instead, we played guys like Big Al, Sap, Howard, Raja, Jefferson, etc. the future has arrived and where are those vets now? Are they part of our rebuilding? Ooops.
I think 90 percent of our defensive issues com from experience/bad habits and Kanter (he's always sucked and always will suck at defense. Too slow).