I think Pop likes aspects of Joseph's game but the kid doesn't pass well and hasn't learned to shoot yet or isn't aggresive / confident enough to find his spots. He's been playing not to lose rather than playing to win since he's been here and doesn't seem to have the drive to get better. At least in the small sample size so far.
To me, he's a single faceted player like Mills was last year but Mills started defending a bit better so he's earned his role. If Joseph was really going to get better, he'd be working on expanding other pieces of his game but I'm not sure he'll ever have the confidence to earn a solid spot on the rotation.
Joseph really should have spent another year or two in college, he's like a bball preemie now, lol.