I think Dok is pretty good defensively, but he can only play for 5 mins at a time. After that, his effort level goes to ****. It was all he could do to run up and down the floor last night. During Summer League, Dok played in 5 - 6 minute bursts, and his energy level was good.
A guy who is 280 lbs isn't going to be able to run and play at the pace the Jazz and Mavs played at last night for more than a few minutes at a time. He just gets run off the floor.
It's not fatigued, although I agree he gets fatigued very quickly. He's been bad the instant he's been on the court both games. He's getting cooked at the rim and is always out of position. It's the preseason, maybe he just had a couple awful games. Or maybe it's that he's not playing in SL anymore and he can't compete with real NBA players. Either way, there are no silver linings about how he's played so far. He's been the worst player on the court both times.