For evidence of his unpredictability, Kelly made major changes to his offensive skill-position lineup this offseason. The most influential move was jettisoning Pro Bowler DeSean Jackson after the best season of Jackson's career. The Eagles also lost veteran slot receiver Jason Avant and traded No. 2 running back Bryce Brown.
To "reconfigure" the receiving corps, which was the mollifying explanation the team chose after Jackson's ouster, the Eagles re-signed Jeremy Maclin and drafted receivers Jordan Matthews and Josh Huff. A trade for versatile running back Darren Sproles also was a headline move.
"It always changes," Kelly said. "I've said it since Day 1, your offense is always personnel driven. So the biggest thing that you have to do is identify the skill sets you have and adjust those skill sets, and that's what football has always been. Our offense has changed every year I've been in charge of the offense, whether I was at New Hampshire, Oregon, or here."
Kelly's pledge since the spring was building an offense that could beat man-to-man coverage. He estimated that the Eagles see more man-to-man coverage than any team in the NFL, in large part because of their pace.
And it's not just the teams that played the Eagles that Kelly considered. He mentioned how the Seattle Seahawks built their Super Bowl-winning defense with big, physical, man-to-man cornerbacks and how the best defenses in the NFL are going in that direction.
"You just can't throw your hands up and say you don't have an answer for it," Kelly said. "We knew that was something that we had to work on and get better on."
Jackson thrived in Kelly's offense, accounting for 30.2 percent of the team's receiving yards and 26.5 percent of the team's receptions. But Kelly only once had a 1,000-yard receiver during his six years at Oregon, with the ball often spread around to different targets and positions.
So replacing Jackson does not necessarily mean No. 1 receiver Jeremy Maclin must compensate for Jackson's 82 catches and 1,332 yards. Instead, it's finding creative ways to split the entire receiving pie. Pass catchers totaled 310 catches for 4,406 yards in 2013, which could increase in Nick Foles' second season in the system. Maclin might be filling Jackson's spot, but Riley Cooper, Matthews, Zach Ertz, Brent Celek, Sproles, and LeSean McCoy are also expected to be major parts.
"I think ultimately, we'd love everybody to have the same amount of catches," wide receivers coach Bob Bicknell said. "If we have seven, they all have 40, and [defenses wondering], 'Where are they going now? How are they doing it? Who's coming into the game now? Oh no, how are we going to deal with that?' If you have different guys creating different matchups, to me, that's pretty exciting."
Kelly walked off the practice field after a late July session and sat down for a radio interview with former NFL offensive lineman Ross Tucker and former NFL general manager Bill Polian. Polian sounded enthralled by the practice he just watched, and asked Kelly about being a passing offense or a running offense. Kelly explained how he wants the personnel to be able to play against any defense, and the Eagles now have a variety of groupings that they can use in different situations.
"I never wanted to be skewed one way or the other, I wanted to be able to handle all situations that are thrown at us," Kelly told Polian on the Ross Tucker Football Podcast. "We can get into a four-wide game, we can get into a two-back game, we can get into a two-tight end game. We feel like we're balanced from that standpoint."
So the dilemma for Kelly is to figure out which seven to nine skill-position players to pick on a given play when only five can take the field. If the Eagles use Sproles and McCoy together with Celek and Ertz, there can be only one wide receiver. And if one of the tight ends comes off the field for a receiver, it still leaves the Eagles without Matthews in the slot.
"That's fine," Bicknell said. "What's wrong with that? Did we win?"
Offensive coordinator Pat Shurmur said the offensive game plan will start with a simple question: "How can we attack them?" With diversity of skills and sizes, defenses are left determining where to focus their attention.
Kelly refuted the notion that Jackson commanded double teams last year and thus opened the field for others. He said most opposing defenses played with single-high coverage and man-to-man for each receiver, with the extra safety focused on stopping McCoy and the rushing game.
That's how the addition of Sproles and maturation of Ertz can help. If linebackers and a safety key on McCoy, then the Eagles could find a potential mismatch with a second running back or tight end.
"Shady McCoy is in the backfield and Darren Sproles is in the backfield - who do you double?" said Eagles assistant defensive backs coach Todd Lyght, a former Pro Bowl cornerback. "Do you double one of the running backs? Well now Zach Ertz has a one-on-one. Riley Cooper has one-on-one, Maclin has one-on-one. It's going to be very dangerous."
When NBC commentator Cris Collinsworth watched the Eagles last season, it reminded him of coaching soccer because of how wide the field was spread. He would see huge gaps of green grass, and then an Eagles player would emerge.
"I find myself many times watching the Eagles, going, 'Where is everybody? Where are the defenders?' " Collinsworth said. "Because there are these big 10-, 12-square-yard areas on the field where these great players are getting one-on-one opportunities to operate."
What stood out to New England Patriots coach Bill Belichick about the Eagles offense in Kelly's inaugural season was their "ability to create huge chunks of yardage" with explosive plays. Jackson was a big part of that - he had 25 plays of more than 20 yards, eight of which went for more than 40 yards - but Belichick said there are other teams with talented players, and the Eagles generate more explosive plays. The Eagles led the league in 20-plus and 40-plus-yard passing plays, were second in 20-plus rushing plays, and tied for the lead in 40-plus rushing plays.
Former Super Bowl-winning coach Tony Dungy, an NBC analyst whose son played for Kelly at Oregon, said Kelly has a way of "magnifying what guys can do and hiding the things they can't do." He did not want to diminish Jackson's big-play ability, but Dungy insisted that the Eagles have other players who can generate big gains.
"I'm not going to say they're not going to miss DeSean, but I don't think they're going to miss a beat," Dungy said. "They're going to continue to improve on offense."
Kelly wants to clear up what he believes are misconceptions: He's not trying to revolutionize football and he has no mandate that the Eagles must run 90 plays, or any predetermined number of plays, per game.
The Eagles showed last season that 90 plays is not essential when they averaged 65.9 plays per game. Although the Eagles' 1,054 scrimmage plays ranked only 13th in the NFL, their average time of possession of 26 minutes, 24 seconds was the lowest in the league by nearly a minute. Football Outsiders ranked the Eagles' 23.38 seconds per play as the fastest pace since the website started keeping track in 1997.
"I know he's still not satisfied with the tempo with which they play," Dungy said. "I know he thinks they can do things better, they can do things faster."
During the spring, Kelly said the Eagles took 10-12 more snaps per practice than last spring. His explanation was that he wanted to get more players more practice snaps. Yet the players were able to go faster because they had a better understanding of the offense.
"We never led the country in offensive plays, nor did I ever care to lead the country in offensive plays," Kelly said. "I never looked at that statistic. We're always, how many points per possession? How many points could we score in a game? Is that enough? That's part of what we always look at."
Kelly has a habit of trying to disambiguate what might seem complicated and complicate what might seem clear. He said he does not think that an offense "can be a thousand miles wide and an inch deep," and emphasized the importance of understanding what he wants the system to be.
The offense might have seemed a success last season, but there will be a new version in 2014. It will come without Jackson and with the addition of Kelly-picked players, and it will be displayed with a scheme that the coach likes to say are just concepts that started in the room with Knute Rockne or Amos Alonzo Stagg.
"We don't do anything revolutionary offensively," Kelly said. "We run inside zone, outside zone, screen play, power play, we have a five-step game and we have a three-step game and we run some screens. We're not doing anything that's never been done before in football."
Maybe not. But the Eagles are trying to find a way to do it better.