Success in the National Football League is often reliant on compatibility between player and system.
When the 49ers hired Chip Kelly as head coach on Jan. 14, the conversation quickly turned to the potential resurrection of Colin Kaepernick's fading career because of that compatibility.
The system Kelly runs, in a lot of ways, seems tailor-made for Colin Kaepernick, enhancing his strengths as a player, while covering up some of his weaknesses.
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The marriage already has defensive coordinators awake at night, terrified of the potential. Three rival defensive coordinators recently spoke with NFL.com's Albert Breer, providing comment on what the pairing of Kaepernick and Kelly could mean.
"I think [Kaepernick] is a good enough passer, but obviously what'll be a nightmare is his ability to run," said one defensive coordinator. "That offense is straight 'Freddy Krueger' when you have a quarterback that can pull the ball and run at any given time."
[RELATED: Mayock: Kelly, 49ers need to take chance on Vernon Adams]
In three years in Philadelphia, Kelly worked with and started three different quarterbacks—Nick Foles, Mark Sanchez and Sam Bradford—none of which posed the threat of run.
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Due to their physical limitations, an entire facet of Kelly's offense was watered down.
NFL defensive coordinators acknowledged this to Breer.
"The fact that Kap can make guys miss and get in the open field, they didn't have that last year at all in Philly," another defensive coordinator added. "We treated [Sam] Bradford like he was under center [at all times]. There was zero threat of him running the ball. We told our guys, 'Don't treat him like he's in the shotgun, he's never gonna pull the ball.'"
Moreover, Kelly said the quarterback situation is what made the 49ers' head coaching job 'attractive.'
"Awesome — could be scary. You get the run threat back to keep the ball on zone read," a third rival defensive coordinator said. "And Kap can throw it deep. Chip stretched the field with [Nick] Foles vertically, and not as much with Bradford. And Kap did a good job with simple reads and progressions early in San Francisco. Chip's intermediate pass game in Philly had those features."
[RELATED: Baalke: Future bright for Kap, but ‘we’ve got two guys’]
Kelly will call the plays for the 49ers, and he figures to have a big say in the ultimate decision at quarterback.
Kaepernick, 28, has run for 1,832 yards and 11 touchdowns in 57 regular-season games.
His single-most dynamic performance in which his full set of tools were on display was the 2012 NFC Divisional game against the Green Bay Packers. Kaepernick completed 17 of 31 passes for 263 yards and two touchdowns, but also ran wild for a record 181 yards, along with two more scores on the ground.
Kaepernick has run for 507 yards and four touchdowns in six career postseason games.