It's all part of a new grievance the NFLPA filed on behalf of Reid -- who believes he's being blackballed for taking a knee during the national anthem in the 2017 season.

Reid met with the Bengals on April 9 and was reportedly asked if he would kneel during the 2018 season. According to reports, Reid refused to answer the question. He ultimately did NOT get a contract offer.

Enter the NFL Players Association which is saying Reid is getting screwed because the NFL has NO league rule prohibiting players from demonstrating during the national anthem and teams that try to stop them are in violation of the Collective Bargaining Agreement.

In fact, the NFLPA obviously references the Bengals in a news release ... though it doesn't call them out by name.

"According to our information, a club appears to have based its decision not to sign a player based on the player’s statement that he would challenge the implementation of a club’s policy prohibiting demonstration, which is contrary to the League policy."

The situation sounds VERY similar to what Colin Kaepernick went through with the Seattle Seahawks -- which shut down a workout with the QB because he would not agree to stop kneeling.

Kaep and Reid are repped by the same attorneys -- Mark Geragos and Ben Meiselas -- who issued this statement:

"Colin and Eric have taken courageous action at the expense of their professional careers and personal lives. They did these selfless acts because they wanted to shine light on inequity and oppression."