I've posted about him many times (started at least one thread).  If he stays healthy, he'll probably go in the teens. He plays the point sometimes for Michigan, but he also has his moments of hero ball (at the same time, he's the only player on Michigan who can create his own shot right now, so that's probably part of it.) I don't think he's got star potential, but could be a very useful player, capable of playing three positions on offense and defense -- sort of like Turner, as you note and I've noted before.  I'd put his ceiling as good Evan Turner with better outside shooting, but bad Evan Turner exists in his game too.  He gambles a little too much on defense of my taste, but I think Stevens can coach that out of him.  Michigan runs a variety of zone defenses, so he'll have an adjustment at learning the various schemes at the NBA level.

The foot injury is a slight worry, but he seems 100% recovered at this point (he rushed back a little too quickly when it was first injured in summer of 2014, and had to shut things down last January, after landing awkwardly on a game-winning basket). 

He is 21 as you note, but doesn't turn 22 until next August, so he's the age of many college juniors despite being a senior.  He's probably done growing, but is very long.  He still needs to thicken up a little.  He's Michigan's best player by a mile (since their other best players are struggling with injuries that look like they will last most of the season), so if he's able to lead them on a tourney run, his stock has the chance to climb into the late lottery.

Pretty bland 3+D prospect. More of a SG than a SF, which lowers his appeal for the C's. I wouldn't call him an awesome defender from what I've seen - looks like he can be above average defensively, but nothing special.

Best thing about him is his lightning-fast release combined with good length for a SG. If his accuracy continues to progress, those traits give him the ability to be a volume shooter even without the ability to create reliably off the dribble.