He tends to have his weight/conditioning issues linked with his defensive ability but there's simply no evidence to support it. There is sometimes an issue with effort on the defensive end, but the numbers say he's a completely average defender
On the whole, I would agree with this.
But I've never had an issue with his defensive capabilities so much as his defensive consistency, and that's where I feel his conditioning has hurt him in a huge way.
Last year when I was watching Sully, his defense always seemed to start out great when he'd first get on the court, and then gradually fade to nothing over time until he sat and rested. You could see that after he'd been on the court for 5 minutes his effort levels dropped to nothing. He'd walk (or slowly jog) back on defense, he'd fail to challenge shots, miss loose balls, have opponents blow by him like he's standing still, etc.
But in the first minute or two on the court, I felt he played with lots of energy - and as a result had a positive impact defensively.
The problem with all this is that now we have guys like Lee, Olynyk, Zeller and Johnson. They might not be the star players, but they are guys who will give you the same consistent energy and effort every minute they're on the court. When you have four guys like that on the team, it gets very difficult to justify putting somebody like Sully on the court, knowing he'll give you good effort for a couple of minutes at best.
I think all four of those guys will get the step over Sully as far as rotations go, and if he's not careful he might even fall behind Mickey.
People think this is insanity suggesting this, but it's not the first time that young guys with clear talent have been benched over effort/attitude concerns. Evan Turner in Indiana. Andray Blatche in Washington. Big Baby in Orlando. Kwame Brown everywhere. Rajon Rondo in Dallas.
You can have all the talent in the world, but if there are guys on your roster who are willing to show a greater commitment than you are, then there's nothing stopping you from either riding the bench or getting waived / traded away.
When you look at guys like Danny Ainge and Brad Stevens - guys who are so obsessed with character, chemistry and intangibles - it becomes all the more likely.