He is - along with his long-term staff, especially defensive coordinator Steve Spagnuola.
Mahomes is good, but he wouldn't have anywhere near his success without these two coaches - and vice-versa. Same with Brady and Belichik. The two complemented each other.
With my limited understanding of the sport, I realize the difference a coach can make. Highlighted right now with Buffalo actually. There is something that sets apart teams that are talented, and those that are talented AND win when it counts most.
Bills need a coaching a reset.