I see I have to make this point yet again.
Developing is one thing. Learning the system is one thing. Getting used to your new coach and teammates and gaining experience is one thing.
HE. CAN'T. HIT. WIDE. OPEN. RECEIVERS.
He can't play football. This isn't an issue of developing, it's an issue of the kid is bad at the game.
NFL QBs don't need to be fluent in a system to be able to make a basic fade pass in the end zone to a top level 6'3" receiver. Not being experienced in a new system has nothing to do with completely overthrowing your deep balls when the WR has two steps on the DB. Lack of system experience doesn't cause a QB to be completely oblivious to open guys when there's little pressure in the pocket. It doesn't cause 75% of his passes beyond the first down marker to be inaccurate to the point of being uncatchable. It doesn't cause him to miss 5 out of 6 throws that should have been TDs.
These are basic fundamentals of being able to play QB. Any NFL QB could do these things on day 1 before they've even seen the damn playbook. He isn't an NFL QB.