The stats will show he regressed, but I think there's more to it. I think part of the problem is that he is trying, or they are trying to make him, fill a role he's not able to fill currently (because they want to know if he can do it). Like in the game vs Toronto they had him bringing the ball up and initiating the offense and he struggled with it, with 6 turnovers. 3 of those were near the end when the game got close, and it exposed another issue - he's not the best decisionmaker under pressure. He had it easier last year when he was coming on with guys like Tatum, KP, Jrue, DW or JB on the court with the amount of gravity they had. Now he's the guy with the most gravity in some of the lineups (Moon gravity, but still).
He tried to do it all himself because he was the most credentialled guy on the court with a bunch of G-leaguers, and he turned the ball over instead of running the plays that (presumably) Mazz wanted him to run. Whether it was lack of trust in his teammates or a desire to win the game on his own (Smartacus syndrome) he made poor decisions to do on his own multiple times. It's preseason - winning the game is important, but it's also important to show you're a team player, and that you can do what the coach wants you to do. If that means pass the ball to someone who might miss, so be it.
I think they tried to put him in positions where he would be uncomfortable, and see how he responded, because this is the best year to do that - there's a number of rotation spots up for grabs, may as well push some of the guys and see if they can do more than what they already have in the past. I think we will see a lot of guys pushed to do that, and they may struggle. They probably want to see if Hauser can be a solid threat as a starter. They want to see how the bigs platoon works out. They want to see if Scheierman can take some pressure off PP and White as a ball handler. So far he has struggled. This may be his ceiling, and he's bumping up against it. He may end up being best as a guy that comes in and spaces the floor and that's it.