He's been more or less the same player as the one I've been watching since his rookie year. His fluctuation in stats are only due to fluctuations in minutes and role. He is what he always has been. Not surprised at all.
The folks who thought he'd made some leap into super-stardom didn't know what they were talking about. He's 27 years old. He already reached his peak.
You know, I recently found a ton of articles that suggest an NBA player peaks at 23 years old. I shared them with this forum and got blasted, because conventional thinking is that they peak at 27. But it's fair to mention that most stars you know of (Kobe, LeBron, Duncan, KG, etc) were pretty much who they were statistically by age 23. Yes, they still improve on individual things here and there... LeBron's always becoming more efficient. But more or less the stats remain the same. Our thinking seems to be wrong. The "27" thing isn't accurate. You'll more or less know what you got by the time a player is 23. Even our own Rajon Rondo was basically at his peak at 23. (13 points, 10 assists, 4 rebounds, 2 steals). The folks who think Rondo is going to make a leap post-injury at 28 don't know what they are talking about.
Jeff Green is another wonderful example. Green's stats as a 22 year old in OKC: 16.5 points, 6.7 rebounds, 2 assists, 1 steal .446/.389/.788
Green's stats now: 16.3 points, 4.5 rebounds, 1.6 assists, .4 steals .448/.385/.781 ... 3 less minutes per game explain the slightly less stats. He's almost identical to the player he was as a 22 year old... he's arguably improved his 3 point shooting, I guess. Well... he takes more of them. I'm sure he's "smarter". But he is what he has been for years.
Good news is that Sully is only 21. He's got a couple years until he peaks.