People should stop saying this. Quarterbacks improve by LIVE GAME ACTION, not by sitting around on the bench. Yes, a year or two as a backup can sometimes better prepare a guy to come in and start than starting as a rookie, but not to the point where you can expect to be able to lean on him to win games for you. The truth is that you SHOULD still expect to have to hold his hand. Hopefully by the end of the year that won't be the case, but for about the first 6-8 games of the year there's no shame in JP still needing to be pampered. He just doesn't have the live experience to be comfortable back there yet.
This is especially true because of the new coaching staff. Here's a guy who, with limited experience, was fighting to get used to the game the way Mularkey & Co. did it. Then you go and bring in a completely new staff and a completely new system, and you expect the kid to not be overwhelmed! He needs more exposure to this system, and more exposure to live game action before we can ask whether or not we should still be holding his hand...because right now the answer to that question is a resounding "yes."