Before left guard Evan Mathis was a member of the Super Bowl champion Denver Broncos, he spent four seasons with the Philadelphia Eagles, including two under former head coach Chip Kelly.

In an email exchange with 9News' Mike Klis, Mathis opened up about his take on Kelly's biggest coaching flaws and how he went from a core member of the Eagles' offensive line to a free agent.

Via USA TODAY:

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"There were many things that Chip had done that showed me he wasn't building a championship team," Mathis wrote in his e-mail to 9NEWS. "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."

After finishing the 2013 and 2014 seasons as the No. 2 and No. 5 offense in yards per game, respectively, the Eagles dropped to No. 12 in 2015. However, the defense never improved during Kelly's three-year run. The Eagles finished no better than No. 28 overall in yards allowed per game, dropping to No. 30 overall in 2015. This not only helps to validate Mathis' point, but it also helps explain why Kelly had trouble finding a defensive coordinator after being hired as head coach of the San Francisco 49ers in January.

As for Mathis' second point, Kelly's relationship with big name players was debated throughout his tenure with the Eagles. While some of his former players, including Mathis, have chosen to speak out against his ability to coexist with big personalities, several current Eagles have publicly disputed those claims. Either way, you can't change the facts -- Kelly traded or released several outspoken starters including Mathis, DeSean Jackson and LeSean McCoy.

Mathis' release in 2015 didn't only have to do with his personality. When Kelly took over full control of football operations last offseason, the framework to a pending contract restructure with Mathis was tossed out.

Mathis couldn't accept that so he took matters into his own hands. After a report surfaced that the Eagles would release Mathis if they couldn't trade him, Mathis told Kliss that he sent Kelly a text message with a YouTube clip asking for his release.

"Since I wasn't going to be paid what I was offered and I wouldn't simply be given better incentives to play for a coach who wasn't building a winning team, I decided I wanted out of Philadelphia."

Mathis has no hard feelings for Kelly, and instead, he hopes that his time in Philadelphia will serve as a learning experience.

"I hope Chip learns from his experiences in Philadelphia and grows as a coach. Maybe he'll find some constructive criticism from this.''

Kelly will have a chance to start over with a brand new roster in San Francisco. Although the 49ers may not have as many personalities as the Eagles or Broncos, Kelly will have his work cut out for him.