I'm hoping that part of the interview process was a discussion on staff philosophy. One of the great things is Grimes has been at a number of successful places and seen a variety of styles of HC and OC. He may very well have a pretty good idea of both the best and worst ways to organize a staff. Has he done it yet? Not entirely on his own. So - if you want to say he doesn't have EXPERIENCE doing it, you've got that right. But to say he doesn't know how to? You can know how to do things without having done them.
Same with calling plays. He hasn't done it yet. This requires a little more nuance than organizing a staff. So I can say I'd have more concern with this too. But again, he's seen a variety of things. And this is an area where someone who HAS called plays can hopefully provide him with some support.