A big reason why the Eagles' defense got so bad so quick

Jimmy Johnson’s most famous line is "how bout them Cowboys" but there’s a line he preaches to this day: "fatigue makes cowards of us all." He is entirely correct, as players tire they make more mental errors.

It’s one of the reasons that Chip Kelly preaches high tempo for his offense, and over his two plus seasons as Eagles coach we have seen, sporadically, how an advantage in fitness has given the Eagles an advantage on the field at the end of games. Grinding a team down so they are worn out in the 4th quarter is nothing new, Chip Kelly just tries to take it to an extreme, and it’s a fine strategy.

But every strategy comes with risk, and tempo’s risk is that your players get fatigued when they have to play a lot. Defenders tire out quicker and more often than offensive players because defenders are always going after the ball carrier. The more they are on the field, the easier they can tire, and the more they tire the worse they play and they run a greater risk of injury. The cumulative effect means that with every game played comes the increased likelihood that a fatigued player will play worse sooner and thus for longer.

In football, a unit’s fatigue is a team effort. An offense that can sustain drives gives the defense opportunities to recover, and a defense that stops teams on third down or creates turnovers gets off the field sooner. The Eagles have struggled in both areas this season.

The offense can’t stay on the field, ranking 6th in 3rd down attempts and 27th in 3rd down conversion rate, meaning they face a lot of third downs and they rarely convert them. In 2013 they were 19th and 12th, they didn’t face a lot of third downs, but they did an above average job at converting them, and in 2014 they were 1st and 9th, they faced a lot of third downs but converted them at a good rate. And the offense has the third most amount of turnovers.

The defense shares just as much blame. They are 24th in third down attempts and 25th in third down conversion rate, meaning they have faced few third downs but have been awful at stopping them. In 2013 they were 24th in both, in 2014 they were 30th and 13th. In their first seven games the defense got 19 turnovers, but in their past four only two.

Another factor is playing time. This season, the entire starting secondary is on pace to play or already has played a career high in snaps.

The Eagles have faced virtually the same amount of plays per game as they did last year, 69.7 in 2014 to 69.4 in 2015.

Jenkins has played the most snaps of all defensive backs (and all defensive players, period) in the league, Thurmond has played the second most. Nolan Carroll has played the 15th most, had he not gotten injured against the Lions he would have played the second most amount of snaps, which would have made Jenkins, Thurmond and Carroll the top three in snaps played. In 2014 Jenkins played the most amount of snaps of any DB and Cary Williams the 3rd, Nate Allen 12th, Bradley Fletcher 18th. In 2013 Cary Williams 3rd, Nate Allen 9th.

These aren’t just high totals, they are needlessly high totals.

In each Chip Kelly’s three seasons, his team has been on the giving and receiving end of a handful of blowouts. In 2013 the coaching staff sat starters late in games. In 2014 they kept them in blowouts longer, and this season are practically refusing to rest starters in games that are out of hand.

We can see a concerning trend. In 2013 the coaches gave backups playing time in blowouts. In three of the four blowouts, every defender that was dressed played. In 2014, with a deeper, more talented defense, the coaches kept the starters in longer in games that weren’t close. And in 2015, they’re basically not playing backups at all. In games where eight or more starters played less than 90% of snaps, the Eagles played well the following week. In games in which seven or more starters played 90% or more of snaps, the defense unevenly performed.

The Buccaneers game is a good illustration of the issue. Down 21 midway through the 3rd quarter, 24 early in the 4th, and facing a short week, the coaches kept the starting secondary in the entire game. Despite Eric Rowe getting more reps in the practices leading up to the game, he played just 6 snaps in dime in the fourth quarter. Despite Chris Maragos playing 43% of defensive snaps prior to the Buccaneers and Lions games, he played no defensive snaps in either the Tampa or Detroit game.

This is a departure from Kelly's time at Oregon, where he and his staff gave backups plenty of playing time. In Kelly's final year in Eugene, senior starters Kiko Alonso, Michael Clay and Brian Jackson all saw playing time in blowouts when they were freshmen under Kelly.

In Philadelphia in 2015, players who are a supposed better fit for the defense than in years past are getting less playing time in similar circumstances. Prior to the Lions game, Eric Rowe was averaging 11.2 snaps per game. In 2013 Roc Carmichael averaged 16.8. The fourth safety this year has been Jerome Couplin and then Ed Reynolds, Couplin played 8 snaps and Reynolds 13, for an average of 2.3 a game, which is a bit misleading because Reynolds played all of his against the Lions and Couplin got 7 of his 8 in the Saints game. The fourth safety in 2013, Kurt Coleman, averaged over twice as many snaps, 4.9.

And it isn’t just an issue with blowouts. In 2013, the defense averaged 7.24 yards per attempt against in the first half of the season, and 7.41 in the second half. In 2014, that split was 7.31/8.26, and so far in 2015, with a nearly entirely new secondary, it is 6.85/7.77.

Not resting starters and giving backups valuable playing time in blowouts offers no solutions and only presents problems. It fatigues starters, puts them at greater risk of injury, and denies young players in-game experience. There’s no upside to keeping Malcolm Jenkins, Walter Thurmond, Byron Maxwell and (previously) Nolan Carroll all on the field late in the fourth quarter of 24 point game. There’s no downside to giving Eric Rowe, (previously) Jerome Couplin and Ed Reynolds playing time in their place in a blowout. There’s no downside to giving Najee Goode playing time in a rout when the linebackers coming off of injuries are getting run all over rather than have him never see the field.

As with everything that has plagued the Eagles in 2015, there’s no simple fix, and a number of issues are not easily controllable by the coaches. But one thing they can do is rest key players in games that are not close to save them for the next week. They're already facing a high workload in close games, over extending them benefits no one.