My view is they won't get any better until Olynyk and Zeller are replaced with more athletic, defensive players up front. They take too big a drop defensively when either is in the lineup. When both are in together, it's a disaster. IT's presence is enough of a defensive liability.
The dilemma is Ainge drafted Olynyk, moving up to get him with the Spurs (Pop knows better),and Stevens is in love with Zeller from their Indiana connect. He is also allergic to rebounders. 
We did not trade up with the Spurs.
It was the Mavs... but can someone tell me what gives with Stevens preference on bigs? We had a soft, almost ethereal presence in our frontcourt and its been a staple since he came aboard. Danny never went to free agency for a scrapper, if Stevens wants to preach versatility someone needs to retort with "oh, that thing your current bigs don't bring every night?" Like, where's the beef dude?
I'm surprised by this persistent myth, and I'm going to speak very frankly.
It's probably not worth getting into a thing, but let one fact stand for many:
Kelly is currently seventh in the league in charges drawn, PER GAME, in only 21 minutes played. Zeller, incidentally, is 18th. Those guys aren't shot-blockers, but they're protecting the rim in the most physical way you can. That's among all NBA players, on all teams.
To remind us, drawing a charge means: getting in front of a 240-lb athlete, moving at high speed, who is about to put his knee into your solar plexus and knock you on your coccyx. A few seconds later you're going to get your ass up off the floor, run down to the other end, and set a screen on him.