The remarkable seasons of Nick Foles, LeSean McCoy, and DeSean Jackson rank high on the list of reasons for the team's success, too.
Good health, however, may belong at the top.
To their credit, the Eagles overcame the preseason loss of wide receiver Jeremy Maclin last year, mostly because Riley Cooper proved to be a suitable replacement, ranking 32d among wide receivers with 835 yards and tied for 13th in touchdowns, with eight.
Once the season started, the most serious injury the Eagles had to deal with was a shoulder issue that kept safety Patrick Chung out of four games. It could be, and should be, argued that it was an upgrade when rookie Earl Wolff played in his place. Including Maclin's 16 games, the Eagles starters missed only 29 games because of injury, according to a comprehensive list put together by the Dallas Morning News at the end of last season.
Only three teams - the Jets, Chiefs, and Redskins - lost their starters for fewer games, according to the newspaper. The Eagles also had 14 players start all 16 games a year ago, the highest number in the NFL.
"Last year, we were just fortunate not to have injuries, and that is just the luck of the draw sometimes," cornerback Cary Williams said.
The Eagles don't think they were just lucky. They also believe they benefited from the way they went about their business. Sports science is preached and practiced at One NovaCare Way. It's as much a part of Kelly's plan as any of his offensive schemes.
"It was a testament to the system we have here as far as taking care of ourselves and being prepared and ready for each and every day," Williams said. "We checked that stuff [in Baltimore] . . . but it wasn't focused on to the Nth degree like it is here. Hydration and how we practiced and the science we put into it, I think it works, and it gave us an advantage to a degree. But you still have to play football and prepare."
It is great that the players all buy into sports science and believe it gives them an edge, but there is still a measure of good fortune when it comes to injuries. They can destroy some of the best teams in the NFL, but it's a great testament to depth when a team can overcome a long list of injuries.
Indianapolis, for example, lost its starters for 83 games last season - only the Giants lost more - and still won the AFC South with an 11-5 record. New England's starters missed 74 games - third highest in the NFL - and the Patriots went 12-4 to win the AFC East.
We do not know yet if the Eagles under Kelly would be able to overcome an avalanche of injury losses, and they are hoping we do not find out.
The Eagles appear to be deeper in some places in Kelly's second season and not as much so in others. Outside receiver, with the departure of Jackson, appears to be the thinnest position on offense. If Maclin or Cooper goes down, Jordan Matthews will likely be asked to step in, and that's a tough assignment for a rookie.
Although the depth of the offensive line is being tested out of the chute by Lane Johnson's four-game suspension, it should become a strength when the second-year tackle returns.
Defensively, the Eagles appear strong up front and better in the secondary than they have been in recent years. The knee injury that ended backup linebacker Travis Long's season before it started, however, could be costly if any of the four starting linebackers is lost for a substantial amount of time.
Long was behind Connor Barwin - at the "Jack" linebacker - on the depth chart, but that role may now fall to rookie first-round pick Marcus Smith. Defensive coordinator Bill Davis said he has a number of contingency plans. He also said Smith is expected to play a "limited role" in his first NFL game Sunday against Jacksonville.
"We have multiple guys that can back [Barwin] up," Davis said. "If you're a backup player, whether it's outside or inside backer, you have to be able to play both sides. Anybody that is up and available can go 'Jack,' and I can change the scheme a little bit to where it doesn't matter, or I can make it matter."
That's good to know, and it will be even better for the Eagles if the offense and defense have to implement a limited number of backup plans again in 2014.