While it certainly remains to be seen exactly what Billy Davis has in store for the Eagles defense, it's obvious that we're in for changes. But just how big will those changes be? The answer may come from a 2009 interview Davis did with Pro Football Weekly while he was still LB coach of the Arizona Cardinals.
The scheme that team ran looked a lot like a 3-4, but was in reality a 4-3 "under" system. It's actually very similar to the Bill Parcells, Bill Belichick, Dom Capers front.
Davis described the under formation as "almost" a 3-4 and his description of it sounds similar to what the Seattle Seahawks were running this season and basically what Pete Carroll did as USC.
The basic idea is that the defense slants to the tight end side. The NT lines up on the center's shoulder on the TE side and the SAM linebacker lines up right over the TE. The DE on that side moves to the shoulder of the OT (this would likely be Cullen Jenkins but maybe Trent Cole in certain situations) , while on the other side, the DE will typically play a 5 technique. This would be where you'd likely see Fletcher Cox.
The other linebacker, which he calls the "predator," lines up on the weakside and is almost exclusively a pass rusher. Davis explained that these guys are typically DEs by trade. On the Eagles, that would likely be Brandon Graham or Trent Cole. He will likely line up in a 2 point stance most of the time, but could easily put a hand down and still have the same responsibilities. This is where the front could appear to be 3-4 or 4-3.
The WILL is the 4th LB in the system, and is more freed up to move around.
Karlos Dansby is the weak-side linebacker. The way the defense is set up, he has a nice protective shield to keep potential blockers at bay. "what we've done with Karlos is put him behind a three-technique, so basically - we call these anchor points - he's got a wall in front of him," Davis said. "So he can run and use his athleticism. The center can't get him because the nose is on him. The guard can't get him because the end is on him. And the tackle can't get him because the predator is on him. So this is your athlete that can run, go cover ground and make plays.
This is where Mychal Kendricks could fit. At his size, you want to keep blockers off him, because when he's free he's athletic enough to run around and make plays.
Where there may be trouble is in the middle. In this system, the MIKE is generally uncovered, meaning that he won't have DTs keeping blockers off him as he would in a 4-3. So you need a big guy in the middle to take on offensive guards.
The only player in the 4-3 'under' who is left uncovered is the "Mike," or the middle linebacker. In the Cardinals' scheme, that's usually Gerald Hayes. "That's my thumper, more of a thick guy," Davis said, circling the capital M on his piece of paper. "In the 'over' front, when i was in Atlanta [2001 to 2003], we put Keith Brooking - we were actually playing an even scheme, too - but we stacked Keith right behind the three [technique] and he got to run and make players and use his athleticism, and he made his first Pro Bowl playing behind the three."
At 6-1, 247 DeMeco Ryans is probably not that guy. However, as you see there Davis mentioned using Keith Brooking in that role and he was only 6-2 240. Gerald Hayes, who played there for the Cardinals, was 6-1, 250. So DeMeco could potentially work in there.
Where the Eagles are deficient now is at NT and strong side linebacker. There needs to be a big space eater in the middle and SAM has just typically been a position of weakness. So they needed to fix that whether it was a 3-4 or 4-3.
Here's how I see the lineup.
-----------3 tech DE-Cole/Jenkins -- NT-? -- 5 tech DE - Cox
SAM ?----------MIKE - Ryans --WILL - Kendricks -- Predator - Graham/Cole
I listed Cole as a 3 technique, which is not his strength, but it seems like that position could be varied depending on whether it was a run or pass situation. Cullen Jenkins is the most natural fit there.
Definitely check out the full PFW piece for further explanation, but if this is the system Davis runs, the personnel may not require as drastic of a change as we first thought.