Missing throws physically, like with inaccuracy, is out of Rees' hands.
But missing even the opportunity to throw to an open receiver due to poor field vision, awareness, or refusal to go through any progression at all comes back to Rees at some point.
Even if you make the argument that some guys are more gifted than others in terms of seeing the field... that comes back to Rees' evaluation, recruiting and work in the film room.
What stands out to me is how often we have receivers open either underneath, or right at their break at the sticks on key downs... and Drew just doesn't get to those reads, almost ever. I get that part of it is forcing the ball to Mayer, because of course you're gonna force-feed Mayer. We'd all be screaming if he had less targets than he does. But Tommy is actually scheming some great stuff (when he isn't ruining his own drives by getting too cute when he doesn't need to), his QB just isn't seeing it.
I also get that it takes time, and Drew has only been at the helm for 5 games... but some of the misses seem like alarming, QB101 type stuff at times.