Every OC talks to his QB up to the 15 second mark. That's such a tired, overblown mantra critics used against Goff and is mind numbingly ridiculous. Reid talks to Mahomes up to that time and nobody makes a big deal out of it.
Now, there have been QB's with such a mastery of an offense that they call plays themselves (Manning, Brady, Rodgers), but even they get input between plays. The big difference is what happens as the QB reads the defense, makes checks at the line, and processes information once the ball is snapped. THIS is where Stafford can make an enormous difference as long as he can digest and communicate McVay's intent as the defense adjusts. Many point to how Stafford will push the ball downfield but that can be disastrous if the line doesn't pick up the rush. It will certainly be interesting and Stafford adds an element to the offense that DCoordinators will have difficulty beating. But they will present problems that will make Stafford look bad at times.
