Sullinger looked so promising his rookie year. If you watch his rookie highlights, you'll see how much skinnier and how much more nimble he was then compared to this year. He was my favorite player on the Celtics that year because he seemed not only like he was going to be good but he also had a desire to improve. It's a shame that it all went downhill from there.
The Celtics didn't blow the pick at 21. Sully was seen as a lottery level talent who could have potentially panned out very well if he put the work in, but he didn't. Sullinger's lack of progression was more on his work ethic than it was talent (unlike say Fab Melo). Sully's panned out better than half the guys that were selected before him and only four guys that were picked after him turned out better than him (Draymond Green, Khris Middleton, Jae Crowder, and Will Barton), and three of those four guys panned out on teams that weren't drafted by them (Middleton, Barton, and Crowder). Not to mention the one guy who we thought the Celtics might regret taking Sully over didn't last in the NBA (Perry Jones III).
I wish Sully all the best. I just wish his best could have been on the Celtics.