James David Ryan, known to the football world as "Buddy," had a long and fruitful career as a no-nonsense, straight-talking assistant coach and head coach in the National Football League.

His former players have called him a groundbreaking genius for his relentless, aggressive schemes. The 1984 Chicago Bears defense, with Ryan calling the shots, set the league’s single-season sack record (72). The 1985 Bears, with Ryan’s 46 defense that overpowered the New England Patriots in a 46-10 win in Super Bowl XX, was likely his fifth symphony.

Former Bears coach Mike Ditka, who often feuded with Ryan during their time together with the team, said Tuesday morning that Ryan "was way ahead of his time."

But when it comes to the Pro Football Hall of Fame, Ryan joins a growing list of former assistant coaches and head coaches who have had a difficult time making it to enshrinement. In fact, coaches who were not former NFL players and have spent the bulk of their NFL careers as assistants have not been enshrined in Canton, Ohio.

Assistant coaches face the biggest uphill climb in the selection process. They are in the same voting category as former players, as well as those who were primarily head coaches, who might have even won a Super Bowl or multiple Super Bowls.

Those are significant hurdles to clear for candidates such as Ryan, who did not have an NFL playing career and did not have the same kind of success as an NFL head coach as he did as an assistant. Ryan’s career as an assistant spanned his time on Hall of Famer Weeb Ewbank’s staff with the New York Jets, including the team’s Super Bowl III win against the Baltimore Colts, to his time with the Bears.

As a head coach, Ryan was 55-55-1 with the Philadelphia Eagles and Arizona Cardinals, and his teams were 0-3 in three playoff appearances -- all with the Eagles. Ryan has been eligible, as a former coach, for election to the Hall of Fame since five years after his exit from the league. He last coached in 1995 with the Cardinals.

He never has been a semifinalist or finalist. With the addition of Tony Dungy in the Hall’s Class of 2016, there are 23 coaches who have been elected to the Hall of Fame, and all had significant careers as head coaches.

The dilemma is that coaches such as Ryan who spent the majority or all of their careers as assistant coaches, do not fall under the Hall’s bylaws as "contributors," a category that removes candidates from the pool that includes former players and head coaches.

In 2014, the Hall’s board or directors changed the by-laws to create a formal "contributors" category. That change meant a contributor will automatically be included among the list of finalists each year.

The contributor, like a seniors finalist, is also voted on, for enshrinement, separately from the other finalists. But a contributor is described by the Hall of Fame as an individual who has "made outstanding contributions to professional football in capacities other than playing or coaching."

So that excludes coaches, head coaches or longtime assistants, for their careers. Some like Bucko Kilroy, who had quality careers as players, but then became longtime, productive scouts, would now fall under the guidelines of the contributors' category.

Dick LeBeau, enshrined as a seniors finalist in 2010, had a Hall of Fame-worthy playing career before his decorated tenure as an NFL defensive coordinator to go with a brief stint as a head coach.

Candidates such as Ryan, because they only coached in the league, do not go into the seniors pool after their 25-year eligibility as a modern-era candidate runs out -- only players go into the seniors pool. Coaches remain among the modern-era candidates, meaning they remain with the growing list of former players. That severely limits their ability to be enshrined if they did not have notable tenures as head coaches.

So, in the end, even with all of the support for what Ryan accomplished as an NFL defensive strategist, he and other highly accomplished former assistant coaches find that an improbable road awaits for them to be enshrined in the Hall of Fame.