Old-school linebackers are the dinosaurs of the NFL | Bob Ford

When Eagles linebacker Mychal Kendricks went to the front office at the end of last season and asked to be traded, it was more than just a player disgruntled with his playing time looking for a new home. It was part of the evolution of the NFL that has seen linebackers, or at least a percentage of them, become endangered species, foraging for snaps like dinosaurs poking at the ice for sprigs of grass.

The true base defense in the NFL now includes at least five defensive backs – nickel coverage – in order to contend with the proliferation of spread offenses in which opponents employ multiple wide receivers, snaky tight ends, and fleet running backs in their passing games.

In the old days, even a decade ago, teams kept three linebackers on the field most of the time (in 4-3 defenses), but the game has changed so dramatically that is now the exception and not the rule. Linebackers aren't going to ever disappear, but the opportunity to get on the field is diminished and, with the Eagles, Kendricks has become the odd-man out, playing just 27 percent of the defense's snaps in 2016.

"You don't have to just win one matchup, you've got to win three matchups when it comes to wide receivers," defensive coordinator Jim Schwartz said. "If they put three wide receivers and a receiving tight end out there, we're going to have to cover them all. … Multi-dimensional players on defense are very important because we don't control substitutions and we don't control what the play call is."

The changing game has also altered the sheer definition of an NFL linebacker. If you looked at the men who played the position and were considered the greatest of their eras, guys who are enshrined in the Hall of Fame, a lot of them wouldn't be on the field much today, or wouldn't be on it for three downs in any case. You can make a list that includes, among others, Dick Butkus, Ray Nitschke, Derrick Thomas, and maybe even someone like Lawrence Taylor, whose specialty was getting sacks and disrupting the offensive line in an era before spread formations and quick-release quarterbacks became the norm.

Linebackers of the past were chosen largely for their ability to stuff the gaps in the line, blitz the quarterback, hit hard, and tackle well. The players on that short list might be either too heavy, too slow, or not gifted enough athletically to adjust to a game in which running the football has fallen so far out of favor.

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"It's always a fight to keep three linebackers on the field because of what the offense presents," Eagles linebacker Jordan Hicks said. "A lot of linebackers are now 230 [pounds], 235. Guys used to be 245, 250, 260. You hear stories of some of the big linebackers in the league and they're pushing them to get down to 235, 240. In order to play three downs, you've got to be able to cover in space and tackle in space."

Like those dinosaurs, linebackers are not disappearing all at once, but the natural adaptations have changed what they look like. The Eagles have two converted safeties – Kamu Grugier-Hill and Nathan Gerry – playing linebacker in camp. Neither is listed at more than 220 pounds.

"All these positions are starting to merge together, because it is hard to find those big bruising linebackers who can also cover," safety Malcolm Jenkins said. "So teams are starting to get a safety who can hold up in the run game and get by with that, like Deone Bucannon in Arizona. He's a linebacker almost every snap. All the lines are getting blurred a little bit, and the game is always changing. It's fun to watch, and it's all about getting the best matchups for your team."

Nomenclature changes slowly, so it might be a long time before the traditional names give way, but linebackers no longer find their top priority at the line of scrimmage, and safeties no longer act as merely last-gasp defenders in the passing game. It hasn't been that way for a while. Now there are hybrid linebackers and "box" safeties, all of whom work in concert to keep up with offenses that get faster and more pass-happy every season.

"I don't know where it will go," Schwartz said. "Things tend to go in cycles. On defense, we have to play what they put out there."

For the Eagles in 2016, that meant putting an extra defensive back on the field for approximately 70 percent of the snaps. (League-wide, according to Pro Football Focus, as recently as 2008 the nickel was used just 43.4 percent.) It also meant seeking out linebackers who fit the modern mold in which pass coverage is often their primary responsibility. The position will always be there. It will just be different.

"Absolutely," Hicks said. "You're not taking linebackers off the field."

Well, not all of them, anyway. Mychal Kendricks would tell you that standing on the sideline does happen a lot these days. He wasn't able to beat out either Hicks or Nigel Bradham for one of the two coveted spots that remain. If it's any consolation to Kendricks, if they were still playing, Dick Butkus and Lawrence Taylor would probably be standing with him at least some of the time.