A year ago, Mathis wanted to rework his deal to remain in Philadelphia. He then reversed course and asked to be released. Kelly eventually acquiesced to that request.
In an email exchange with Mike Klis of KUSA-TV in Denver, Mathis blasted Kelly's scheme and his handling of star players, specifically noting that Chip couldn't have handled the personalities on the Broncos' roster.
"There were many things that Chip had done that showed me he wasn't building a championship team," Mathis wrote. "Two of the main issues that concerned me were: 1. A never-evolving, vanilla offense that forced our own defense to play higher than normal play counts.
"2. His impatience with certain personality types even when they were blue-chip talents. The Broncos team I was on would have eaten Chip alive. I don't think he could have handled the plethora of large personalities."
Mathis' indictment of Kelly's scheme seems to run counter to the widely held belief that the former Oregon coach is one of the most creative, modern minds of the game.
Mathis added that he hopes Kelly learned from his failures in Philly, especially handling players.
"I hope Chip learns from his experiences in Philadelphia and grows as a coach," he wrote. "Maybe he'll find some constructive criticism from this.''
While it's clearly a one-sided criticism of Kelly, Mathis' note meshes with the critiques of many -- but not all -- on how the coach handled star players in Philadelphia. It's often been said that the most difficult job of a professional coach is managing personalities of players getting paid millions of dollars.
Mathis told Klis he hasn't decided if he'll retire -- he's set to be a free agent -- but plans to train as if he'll continue playing.
As for how Mathis' days in Philadelphia ended, before being cut in June the guard said he texted Kelly a link to the YouTube video of Engelbert Humperdinck "Please release me" in March.
Is there any chance Chip Kelly cracked a smile at that brilliant text?