A lot of the time where gallup was open on 9 routes were double moves.. i remember times where dak overthrew him or threw the ball out of bounds when he was wide open no one near him no slowing down... i watch the games.. the film is also on youtube.. im not here to argue or **** on dak because i am a fan of both players. I just like to call it how i see it
Edit - the saints game really comes to mind regarding this topic.. also the eagles game
It's been said above, but the WRs are taught to keep their heads down on their routes and to look for the ball at the top of the route where they expect it. Cooper, at least, said a couple of times last year that he'd made the mistake of looking up too soon and losing speed. It might not appear that they're losing speed as they're still running full-out, but it slows them down.
That doesn't meant that Dak's not also overthrowing, but it's interesting that he had such good downfield chemistry with Cooper and was a lot worse with Gallup (though they were very close a lot of the time). It suggests, at least, that the WR is involved in whatever the problem is and that it's not just the QB.