Juan Castillo is solely responsible for the Eagles' struggles on defense. It has nothing to do with the inexperience at linebacker or the poor tackling by the secondary - at least, not directly. No, the problem with the Eagles defense is a catastrophic snowball effect caused not by Castillo's unfamiliarity with defensive coaching, but by his own vanity.
When you are just hired as a head coach or offensive or defensive coordinator in the NFL, you have two approaches to putting your mark on the team: the "Morris" approach and the "Harbaugh" approach. The Morris Approach refers to Raheem Morris, head coach of the Tampa Bay Buccaneers. When he got hired, he brought in an entirely new system from Jon Gruden's. He needed different players than were on the roster and so he started from scratch: drafting a quarterback and linemen to build the team from the ground up. Management in Tampa knew what they were doing and they knew there would be some growing pains. And after one poor season, the Bucs went 10-6 and now are currently two games ahead of the Philadelphia Eagles at 3-2.
The other approach, the Harbaugh Approach, owes its name to Jim Harbaugh. Management in San Francisco took a completely different angle on the matter. They hired a coach they knew would work with existing players on the roster to bang out an effective scheme. Notice that even though there was a regime change in San Francisco, the roster really didn't change all that much. Sure, they drafted Collin Kaepernick in the second round, but he's found a nice role warming the bench so far. With a new coach and the same players, the 49ers are sitting pretty on top of the NFC West at 4-1 and are 3 games ahead of the Eagles. In both approaches, management and coach are on the same page - either the coach is willing to work with the current roster or management is willing to allow the coach to dismantle and rebuild the team.
This is where Castillo's vanity comes into play. When he was hired, he wanted to put his own stamp on the Eagles' defense and opted for a zone coverage scheme. That's fine, but when Howie Roseman was in the process of acquiring Dominique Rogers-Cromartie and Nnamdi Asomugah - two very good but strictly man corners - it was Castillo's responsibility to put his foot down and say, "No, I run a zone scheme. These are two good players, but they don't fit well into what I want to do." If Howie didn't back down, then Castillo would have to change his plan. He would have to play with the hand he's dealt, so to speak. He knew that he had three cornerbacks whose skill set was inclined towards man coverage, not zone. But Castillo became convinced of the effectiveness of his own scheme and allowed the past five weeks of football to happen. Each week, the passing touchdowns pile up; each week, the inexperienced linebackers get gashed on the ground as they're forced to focus on both playing the run and zone coverage. And what were the words out of Castillo's mouth before the Bills game?
"Each week, we get better and better."
Does this man honestly believe that? The Eagles have given up points when their opponents are in the red zone over fifty straight times. Jimmy Johnson - the man responsible for making the Eagles' red zone defense the stiffest in the league - is doing cartwheels in his grave right now. And yet, here Castillo sits, smug with his scheme and his job, telling the world that the Eagles' defense is getting better each week. All over the league experts are pleading with the Eagles to let Nnamdi, Dominique, and Asante play man while the rest of the team bottles up the run. But Castillo won't do that, because it's not his scheme.
No, it's not your scheme Juan. But in case nobody has told you yet: your scheme sucks. You've mismatched personnel with philosophy, and the Eagles are dying a slow, painful death, because the offensive turnovers leaves your horribly coached unit on the field. Until you admit that you are as human as you so quickly admitted Nnamdi is, and force a change, the Eagles will lose. And you will be fired.