PHILADELPHIA -- By definition, defensive football is reactionary. Bill Walsh pioneers the pass-first, short-route West Coast offense, so Buddy Ryan comes up with the radical, high-pressure 46 defense.

It’s the circle of football life.

In the modern NFL, with so many varieties of offense out there, defenses have to be versatile. That’s especially true for a team like the Philadelphia Eagles, who are trying to implement an entirely new approach with half a lineup’s worth of new starters.

“It’s definitely a 3-4,” linebacker Connor Barwin said. “Well, it’s more of a 3-4. It’s a hybrid. It’s more of a hybrid than I played in in Houston. It’s more of a hybrid than I’ve seen in other defenses. We went four down, we went three down, we went normal 3-4 base, which is really like five down.”

That was against the Washington Redskins, with the normally mobile Robert Griffin III and the run-first approach built around running back Alfred Morris.

So what will the Eagles' defense look like against San Diego Chargers coach Mike McCoy’s pass-first offense featuring the decidedly less mobile Philip Rivers at quarterback?

“It’s a completely different challenge this week,” Barwin said. “It’s that West Coast offense. Their run game is inside the tackles. Last week, their run game was outside the tackles. The quarterback sits in the pocket and gets the ball out on time, which is the opposite. Last week, the quarterback ran. They ran bootlegs and play-action. San Diego is completely different.”

So the Eagles' defense will look completely different. Or it will look the same and then behave differently. Much of what coordinator Bill Davis has his players do is reliant on their reading the offense and reacting to it.

“The scheme is built to where any member of the defense can be blitzing at any given time,” Davis said. “We have blitzes for every position -- corners, safeties, nickels, dimes, Mike ‘backers. Anybody can be a blitzer, either through an active call or a check.”

Cornerbacks Cary Williams and Brandon Boykin each blitzed Griffin on Monday night. So did linebackers DeMeco Ryans and Mychal Kendricks.

“A lot of people thought we blitzed a lot against the Redskins,” Barwin said. “I don’t think we blitzed very much at all. They max-protected. They kept in six, seven guys to protect and only put out three receivers.”

When that happens, Barwin said, the defense reacts to it. Ryans was assigned to cover a running back on one play. When the back stayed in to pass-block, Ryan rushed the passer. He got a sack on the play.

“You don’t stay and cover grass,” Barwin said. “It looks like a blitz, but I don’t think we really dialed up that much.”

The Eagles face some different challenges in Week 2. They have historically had trouble with tight ends, and Antonio Gates is among the more dangerous ones. Rivers also throws to 6-foot-5 wideout Malcolm Floyd, who might be covered by the 5-9 Boykin.

It is a very different puzzle from last week’s. It will be interesting to see how Davis reassembles the pieces.