2 Chainz's Ex-Manager Reveals Why Nike Picked Him for Kaepernick Ad

"Lose 120 pounds and become an Ironman ... after beating a brain tumor."

Damn impressive, right?

Colin Kaepernick thought so ... and that's why Charlie Jabaley was picked to be featured in Kaep's new Nike commercial.

Charlie used to be 300 pounds. He also used to manage rap stars like 2 Chainz and Travis Porter -- but quit his job to focus on health when doctors told him a brain tumor from his youth had come back.

Charlie tells TMZ Sports ... he had always wanted to be a pro athlete -- and after his health scare, he kicked his workout regimen into high gear.

He started biking. Running. Swimming ... and documented the whole thing in a fan-made Nike commercial.

The spot went viral and caught the attention of Nike execs who were so impressed, they selected Charlie for a feature spot in the new Kaep ad ... alongside LeBron James and Odell Beckham.

"They put me in what I believe is the most culturally shifting ad campaign of all-time," Charlie tells TMZ Sports.

"It's just an honor to be in it. I love it. It's so inspirational. It's talking about the crazy dreamers of the world, and that's all I am. I'm one of the crazy dreamers. So thank you, Nike."