ONE REASON the 2013 Eagles improved from 3-5 in the first half of the season to 7-1 in the second half was the progress made by Bill Davis' defense. It still had issues, some of which were exploited in the 26-24 playoff loss to New Orleans, but the trend was positive; most of us thought Davis did an excellent job scheming to hide weaknesses.

This season, as the defensive line grew into a dominant group, and the pass rush from the line and the outside linebackers gained consistency, for quite a while it seemed that the unit's overall progress would continue. But like so much about the 2014 Eagles, that progress was exposed as a mirage, in a trio of deadly December losses that swept away the team's playoff hopes, rendering meaningless Sunday's season-ending visit with the Giants. After going 6-2 in the first half of 2014, the Eagles can't possibly do better than 4-4 in the second half.

"Our Achilles' heel all year was the 'X plays' [of 20 or more yards], and that's something we didn't get fixed," outside linebacker Connor Barwin said this week. The Eagles' 66 20-plus-yard pass plays given up lead the NFL. "It's kind of a tough question, because usually teams that give up the X play can't get to the quarterback, and we've done a good job of getting to the quarterback, and [still] given up the X plays. It's been an issue for us all year. We were able to kind of cover it up, or play past it, or play over it for three-quarters of the season. In the last month, we weren't able to."

Safety Malcolm Jenkins, like Barwin, said he thought the fatal flaws were present all along. Asked whether he felt the season slipping away the past few weeks, Jenkins said: "The biggest thing was, especially early in the year, we got a lot of wins that probably we shouldn't have gotten . . . we never corrected the problem . . . If we continued to play the way we did, eventually it was going to catch up with us, and it has. We had the talent and the mental toughness to really overcome a lot of the mistakes we made, but as the season goes on, more often than not, when you gamble, you're going to lose. It's some glaring mistakes that we've got to clean up if we want to be a playoff-caliber team."

Davis, the defensive coordinator, said both the X plays and the persistent drive-extending penalties obscured areas of improvement. Asked whether he thought his defense got better in 2014, Davis said: "In a lot of categories, yes, and in a very important one, the deep pass, the vertical ball, the plus-20 passes, we're not. I've got to get that fixed. We're not better in our penalties and our discipline, so those are two areas that have hurt us, that we have to get corrected, that we have to make sure that we look at every aspect of why it's happening, like we have all year, but the deep-pass ball has got to stop, and that's what's giving up yards and it's why our points [allowed] aren't where they need to be.

"There are a lot of aspects of the defense that have vastly grown: The run defense, you know, our run average per attempt is high [3.8 yards, sixth-best in the league], our takeaways are in the top five, our sacks are in the top five, our opponent completion percentage is in the top five. But the one that matters most is points, and that's the one we've got to get down, and right now we're not doing that. That's a product of the deep passing game and the penalties."

Jenkins agreed with Davis about improvement being overshadowed.

"Especially [compared with] early in the season, I thought we made a lot of strides in our running defense, we made a lot of strides in tight coverage [nearer the line]. We also made a lot of strides in just taking the ball away," Jenkins said. "Where we've declined is penalties and the deep balls over our head . . . You see us dominating throughout the whole game, then we give up one. And that's really been our Achilles' heel, that and the penalties on third downs, giving people multiple opportunities to beat us."

Offenses lately have done a good job of figuring out exactly when the Eagles' cornerbacks, especially Bradley Fletcher, have single coverage on playmaking receivers - even if Davis gives frequent safety help. (Of course, often the safety help is Nate Allen, who sometimes gets there and sometimes does not.)

Jenkins said "there are some things you aren't going to be able to get to," playing single high safety, as the Eagles often do, and "there is an art in putting yourself to the ball" instead of guessing wrong and going to help out where the ball isn't headed, but "there's no coverage where everybody has help. If you lean one way, everybody else on the other side has to win their matchups. That's what it comes down to, is just winning your matchup."

Jenkins said some of the defensive penalties have been aggressive penalties "we'll have to live with," given that the Birds, for example, try to hit the quarterback's arm as he's throwing, which can easily lead to striking the helmet. Jumping offside or getting a personal foul for a shove after the whistle is different.

"The penalties that we can control, that's just a lack of focus and a lack of being in the moment and knowing how precious those snaps are. Those are selfish penalties," Jenkins said. "Those are one thing we can't have and we've got to clean up."

Allen said "there's no special recipe" to fixing the penalty problem.

"Just being smart on third downs. Whether it be not grabbing guys, not getting into it with guys, watching the snap, whatever it may be," he said.

Davis said he thought he'd tried everything to stop the deep passes, at least from a schematic standpoint. (You've heard Davis' reasons for not replacing Fletcher, the focal point of the longball meltdown - he thought trying to restore Fletcher's confidence was a higher percentage move than tinkering with his nickel coverage by moving Brandon Boykin, or messing with his dime by moving Nolan Carroll.)

"We looked at it all year, and schematically, I'm telling you - are we splitting the safeties? Who am I giving help to? Who's having these issues, is it schematic? Is it calls? Is it timing of calls? Is it technique at the line? Is it technique deep? Is it underneath coverage? Is it supporting the deep coverage?" Davis said. "There are so many things we're looking at and wracking our brains and trying to figure it out and solve the problem, and have been all year.

"It's kind of like the turnovers on the other side - you put all your focus on it and you're trying to fix it. We've got to find ways to fix it, and we will. We will."

Just not in time to save this season.

On Twitter: @LesBowen

Blog: ph.ly/Eagletarian