The gripes from fans have been: He doesn’t work hard enough. But if the goal is fat loss, exercise shouldn’t be the primary means to achieve that goal. Diet is. I wouldn’t have to touch a dumbbell or a button on a treadmill for eternity, yet I could effortlessly quell excess fat gain as long as I wasn’t exceeding my own personal glycogen levels.
The Celtics, or Sullinger for that matter, apparently fail to understand this. According to Sullinger himself, he spent the 2015 offseason working out every day and all day - an absolute foolish and clueless means of achieving optimal health as extended elevate heart rates do not allow for any recovery, destruct hormone levels, drain glycogen levels which spike sugar cravings, and lead to overtonis in the muscle cells. There is literally nothing to be achieved out of what Sullinger was doing this past summer. Cliches make Americans believe that the harder one works, the more they gain. However cliches are not based upon science and the science in this leads completely the other way.
A few things I don't agree with though. The glycogen part at the beginning is odd. That's just glucose, which is your body's preferred source of energy. However, you can easily implement a keto type diet (cyclic ones probably being the best), where your body will use ketones for energy use, since it's completely depleted of glucose, and you will lose fat. Perhaps that's not optimal for muscle fullness (your body can hold roughly 2.7 grams of water per gram of carb) or performance, but for sheer fat loss it works very well for a lot of people.
Sullinger is a classic endomorph. He needs to watch his carb intake, as he is likely carb resistant, and implement carbs (of the low glycemic variety) in a peri-nutrition style (pre, post, and during training). When he's not active he needs to eliminate carbs and instead have good fats (no sat or trans) and veggies (obviously along with protein) since fat and fiber slows down the rate of absorption.
This isn't rocket science. It's very simple, but you need to track your macros, weigh foods, time your meals, and have a plan. It's called being a professional.