Kap's longtime friend and former teammate sounded off on social media late Wednesday night ... responding to a quote from Jay that read "... everybody knows I agree with what you're saying [in Kaepernick's underlying message]. So what are we gonna do? ... [Help] millions and millions of people, or we get stuck on Colin not having a job."

"These aren’t mutually exclusive," Reid wrote back ... "They can both happen at the same time! It looks like your goal was to make millions and millions of dollars by assisting the NFL in burying Colin’s career."

Of course, Jay-Z has taken plenty of heat for his partnership with Roger Goodell -- one where Hov will help the league with entertainment and social justice issues -- because some feel it undercuts Kap's cause.

Clearly, Reid is in that corner ... because he also retweeted several tweets that hammered Jay for the move, including one that read, "You don't 'move on' from riding for the people."

For his part, Kaepernick has yet to respond directly to the whole ordeal ... but he did write on social media Wednesday, "Today marks the three year anniversary of the first time I protested systemic oppression. I continue to work and stand with the people in our fight for liberation, despite those who are trying to erase the movement! The movement has always lived with the people!"