Reid is a former Pro Bowl safety who racked up 67 tackles and 2 interceptions last season -- and many believe he should have easily gotten a roster spot in 2018 based on his talent.
Bleacher Report had Reid ranked as the 10th best safety in the league in 2017.
A source with knowledge of the situation tells us ... Reid's grievance invokes Trump's name -- essentially claiming POTUS has intimidated owners into not signing players who kneel.
In fact, one NFL source described it as the "Bezos Effect" -- meaning owners are afraid if they cross Trump, he'll publicly attack them like he did to Amazon because he doesn't like Jeff Bezos.
Trump has regularly trashed players who took a knee during the anthem -- in fact, he just went after them during a rally in Michigan last week.
The NFLPA has issued a statement saying, "Our union is aware that Eric Reid and his legal representatives filed a collusion claim, which will be heard through the arbitration process as spelled out in our collective bargaining agreement."
"Our union supports Eric and we are considering other legal options to pursue."