One dynamic that has generated considerable reaction since Chip Kelly arrived was the merits of a 3-4 defense or a 4-3 defense, and with good reason. The Eagles have played a 4-3 defensive alignment for much of recent memory, and their roster had been assembled with a 4-3 in mind. But it's becoming more likely that Kelly moves the Eagles to a 3-4, which would require some positional and roster changes.
It's first important to understand that any defense will succeed or fail based on personnel, and the players are far more important than the system. Plus, the team must have the players to fit. But for the sake of discussion, let's look at two factors -- the teams that run a 3-4 defense in the NFL, and the key pieces the Eagles possess.
3-4 IN THE NFL
There is no one way to measure the best defenses in the NFL, but here are the top 16 scoring defenses, and the system they use:
Who is the best fit for the Eagles at defensive coordinator?
1. Seattle Seahawks -- 4-3
2. San Francisco 49ers -- 3-4
3. Chicago Bears -- 4-3
4. Denver Broncos -- 4-3
5. Atlanta Falcons -- 4-3
6. Pittsburgh Steelers -- 3-4
7. Miami Dolphins -- 4-3
8. Cincinnati Bengals -- 4-3
9. Houston Texans -- 3-4
9t. New England Patriots -- 4-3
11. Green Bay Packers -- 3-4
12. Baltimore Ravens -- 3-4
12. New York Giants -- 4-3
14. Minnesota Vikings -- 4-3
14. St. Louis Rams -- 4-3
16. San Diego Chargers -- 3-4
So 10 of the top 16 defenses (thus, the top half of the league) use the 4-3 alignment.
However, not all defenses are the same. Some use different blitz schemes, or line up players in differences. Tampa has a 4-3 front, but it uses different principles than other 4-3 defenses. And some defenses are hybrids of different schemes.
Of the past 10 Super Bowl champions, five have run a 3-4 and five have run a 4-3. Both defenses in this year's game run a 3-4. So there's no clear consensus around the NFL.
Yet the best defenses have players that fit the system. That leads to the players on the Eagles roster...
KEY PLAYERS ON EAGLES DEFENSE
There are positions that are critical in a 3-4 defense, such as a space-eating nose tackle and pass-rushing outside linebackers. The Eagles lack the nose tackle, and that would be a requirement if they make the switch. But here are five key pieces on the Eagles front seven, and how they fit:
FLETCHER COX -- The most promising player on the Eagles defense is Cox, the 2012 first-round pick who had a strong rookie season. When Howie Roseman talks about the Eagles featuring versatile defenders, that list starts with Cox. During the pre-draft process last season, Cox was seen as a potential fit as a five-technique defensive end in a 3-4 or a disruptive defensive tackle in a 4-3. So he'll be able to make the switch.
MYCHAL KENDRICKS -- Kendricks actually played in a 3-4 defense in college. He was an effective pass-rusher and had experience as both an inside and outside linebacker. He was viewed as a potential 3-4 defender in the NFL. So Kendricks is another player who could make the transiton.
DeMECO RYANS -- Ryans' case is perhaps the most interesting. He's viewed as a prototypical middle linebacker in the 4-3, and was not as effective in 2011 when the Texans transitioned to a 3-4. Of course, he was also coming off major surgery, but Roseman cited the scheme change when the Eagles acquired Ryans. That could have just been a talking point, but the offseason will be critical for Ryans if the Eagles make the switch. He's one of the Eagles' most consistent players, and he's a valuable player in the locker room. But he also has a big salary, and the Eagles must hope that the decline in 2011 was more the result of the injury than the scheme change.
BRANDON GRAHAM -- Similar to the Eagles draft picks in 2012, Graham's pre-draft hype in 2010 focused on his potential as a fit in the 3-4. A greater proportion of NFL teams play the defense than college defenses, so there is much about projection about the transiton of college pass rushers without much evidence. Graham's size and his skill set made him an option before the Eagles drafted him and the point was moot. Graham would need to learn a new spot, but he does have pass-rushing skills. The Eagles hope they can salvage this pick, and a fresh start in a new scheme might benefit Graham, who had a hard time with the Wide 9.
TRENT COLE -- At 30, Cole is not a long-term piece for the Eagles. But expect him on the roster next season by virtue of the money he's owed and his past performance. His entire NFL career has been in a 4-3, though. He was viewed as a potential linebacker when he was drafted in 2005, but Cole has added weight since then and he's been a down rusher. The Eagles would need to see how he fits in a 3-4, although it's not uncommon for players such as Cole to move to a 3-4 later in their careers. Like Ryans, the offseason will be critical for Cole if the Eagles make the switch.