Chip Kelly is clearly changing some of the strategies he used at Oregon, which is smart. The NFL plays a very different game, and a good coach needs to adapt.
However, Kelly’s program was never just an offense, based on one type of (no huddle, read option) play. He developed a system whose different elements reinforced each other. We’ve seen, for example, how his focus on special teams influences his roster choices on offense and defense.
That’s great when one part of the team reinforces another. Defensive backs who tackle well in space also excel on special teams, for example. But it works in reverse, too; changes can cause unintended problems.
In his four years at Oregon, Kelly got the nickname "Big Balls Chip" in large part because he went for it on fourth down 118 times, converting 71 of those (60.2%). This, according to Cfbstats.com
Obviously this gave the Ducks a lot of offensive momentum when they succeeded, and it reflected a lot of faith in the ability of his defense to stiffen up if they didn’t. The Ducks ranked in the top ten colleges for red zone defense. This strategy also reflected the Ducks’ lack of a reliable kicker, which obviously changes your calculation.
But there is another advantage to Kelly’s fourth down aggressiveness, particularly in his system: it gives his defense a chance to rest. This is true if they get the first down obviously, but it’s just as true if they don’t; your opponent will have a shorter field, so they’ll either score or kick it away in short order. With a powerful offense, Chip has always been confident in a shootout. Stop them or don’t, let’s just get the ball back and score again.
The Chip Kelly Eagles have faced 21 fourth downs, and have only gone for it once. They tried 7 field goals, making 5, and punted 13 times. The Ducks went for it 30 times a year, in a 13 game schedule; Philadelphia is on pace for 5. The only fourth down attempt this year came in game one, on his first drive against the Redskins, and it worked to perfection, extending the drive, building confidence and momentum, and keeping the defense off the field. All this kicking has the opposite effects. One writer thinks Chip's fourth down caution cost him the game against Stanford last year, too.
It’s not clear why Chip is kicking more, but I suspect he’s being cautious, falling prey to that typical NFL conservatism. But his system needs fourth down aggressiveness to pump up the offense and spell the D. The defense played beautifully against Kansas City, with a lot of heart and effort – until the crucial 8 minute drive in the fourth quarter, when they ran out of gas and couldn’t stop the Chiefs on third downs. Against the Chargers, too, the defense couldn't muster the energy to stop the Chargers' last second, game-winning drive.
When Chip Kelly's system is working, he wears out opponents and runs away with games at the end. By playing it safe in Philadelphia, he's setting up opponents to do the same to his Eagles.