He finished the season as the NFL's leader in carries (314), rushing yards (1,607), and yards from scrimmage (2,146). McCoy had been one of the NFL's elite running backs before Kelly's arrival, but the marriage between his skills and Kelly's system is a potent combination.
"You look at LeSean - he's a space player," said running backs coach Duce Staley, who is fifth on the franchise's all-time rushing list. "You want to get him in space where he has a matchup where he has an advantage. So this offense is all about space - creating space and putting playmakers in space. That's why it's a perfect fit for him."
Staley said that if you were to design a running back for Kelly's offense, that rusher would play like McCoy. He thought of the number of running backs who could do all that McCoy does on every down. Staley held out his right hand and said they all can be counted with those fingers.
"What he brings to the table - being able to make people miss in small areas, being able to pick up the blitz, be able to run a pass route - he has all the tools," Staley said.
When Kelly accepted the Eagles job in January 2013, he knew of McCoy from highlights and reputation. But he never studied McCoy. When he watched the film, he marveled at McCoy's ability to make people miss. This is a talent that McCoy was born with and likes to flaunt - his Twitter handle is taken from the phrase "cut on a dime" - and it is especially effective in space.
McCoy made sure to mention that the offensive line is the reason for his success in Kelly's offense. To his point, there was a clear difference when he ran behind a healthy offensive line last season. But the linemen can only open the holes or block down the field. It's what McCoy does when he goes through those holes or is in open space that distinguishes him from other running backs who have played for Kelly.
"It starts off with [the offensive linemen] and then having so many one-on-one opportunities," McCoy said. "And just the ability to stop on the dime, have different cutbacks. I think the best thing about my game is my vision. I see things happen before they do."
Cutback ability and vision are the skills that former Oregon star running back Kenjon Barner pointed to as essential for Kelly's offense. Barner rushed for 1,767 yards for Kelly in 2012, the most of any player during Kelly's six years with the Ducks.
"You have to have vision in this system," Barner said. "If you can't see lanes, if you can't see cutbacks, this offense won't be good for you."
Barner cautioned against designating the running back's success as a by-product of the system. He said that "cut on a dime" ability is what separates McCoy from every other running back in the NFL and predicted that McCoy's yards after tackles and missed tackles would support this opinion. Only Minnesota's Adrian Peterson and Seattle's Marshawn Lynch had more of both, according to Pro Football Focus.
Those two running backs are also some of the few three-down running backs in the league - a running back must be able to block to play on third downs. When Staley was asked what skill allows McCoy to thrive in Kelly's offense, he identified blocking.
"If you can't block [on] third down, you have to come off the field," Staley said. "Blocking is the one thing he takes pride in [because his other skills come naturally], and he works on it."
After Kelly came to Philadelphia, it was McCoy's ability as a blocker and some of the ancillary parts of playing the position that caught his attention. McCoy was targeted in the passing game as the season progressed, and Kelly became confident in McCoy's route-running ability. Kelly was struck by an option route in Week 2 that was supposed to go for 3 or 4 yards and turned into a 70-yard gain.
Kelly said he saw the highlight runs and had an idea of what McCoy could do as a rusher, but "his ability to be a complete back is really what's impressed me."
Before last season, McCoy's career high for carries was 273. Staley admitted that he did not think McCoy could be a 300-carry running back and that McCoy "proved me wrong."
Kelly has pushed McCoy throughout the summer, and McCoy has welcomed it. McCoy reiterated his respect for Andy Reid but said that he's never had a coach push him like Kelly.
Despite McCoy's impressive numbers last season, Kelly believes McCoy can still do more.
"I don't think he's complacent or happy in terms of where he is," Kelly said. "Hopefully, he thinks that last year was just him scratching the surface in terms of what his potential can be."