Some coaches are good enough to teach a scheme or system to their players and be successful in the NFL. Some coaches are capable of taking raw player talent and develop them into better players...and some coaches can do both, teach a scheme or system and improve their raw talent.
But some coaches are not capable of doing either...they rely on getting the best talent their team management can buy on the free agent market or draft and hope those players can perform the scheme well enough to make the coaches scheme look good.
Right...and some coaches are handed Tommy Togiai and a few rookies and are expected to stop the run.
Just like you have students who are mentally challenged and just don't understand the material, you also have players who are not NFL football players even if they are on a NFL team.
