I realize a lot of this is driven by excitement in a qb starved organization paired with a slow NFL news cycle now that the draft is over. But man, people need to get a grip. Let the process play out and get to training camp before everyone loses their mind over whether or not Fields is ready to start. Give the guy a chance to learn the offense, execute the offense, and -show- what he can do and why he deserves to be that starter.
I honestly think Nagy's coach speak right now is the correct message. It gives both the team and Fields time to get acclimated to each other and will let Fields' play dictate the terms of his starting. The only way they royally screw this up is if they don't give Fields reps in practice as like the 3rd string behind Foles or some crap like that. That would be a complete disservice to both Fields and team to not give him the opportunity to show what he can do; however, there are no indications that is the case. So, I am going to assume positive intent that Dalton will get reps with the 1's, Fields will get reps with the 2's, etc. One of the keys right now in the development is just giving him reps to work through the transition from college to pros and making those adjustments to the NFL game. Give him that chance, and we will see what he does to earn reps with the 1's and eventually to start.
And as far as history with other qb's goes, imo, it doesn't matter as far as the exact timing - ie, which game a guy becomes the starter. You should not start or sit a guy by game X based on what some other team did with a different roster at a different point in time. As I said above, you give the guy an opportunity to show what he can do. Have your checklist of things you need to see him do to determine he is ready. When the checklist is complete, you start him. If that is game 1, you start him game 1. If that is game 5 and you current starter is just kinda meh, you start him game 5, etc. Just don't do the what I thought was kinda dumb like the Dolphins did last year and start your guy (in their case Tua) because you predetermined you would start him after the bye week regardless of how your team is playing, being in a playoff hunt, or that qb's readiness level.