As a NFL head coach, what do you need to really do? Hire the good offensive and defensive coordinators, manage the media, communicate with the front office and manage the game on Sundays.
The head coach does not need to be the smartest X's and O's guy...that needs to be the coordinators. The head coach doesn't need to know the perfect techniques of each position, that is the position coaches job.
In my opinion, a great NFL head coach needs to be a great leader and one that communicates great on all platforms between fans, media, front office, coaches and players. Get the right coordinators and position coaches then the head coach should be more of a figure head than anything.
Why Saban and Urban among others would struggle in the NFL is that they have a drive for perfection/undefeated seasons and micromanage the game. If perfection isn't achieved, then essentially spontaneous combust. Did you see Saban's reaction when his RT had back to back penalties to end the first half? He erupted.