Ex-NFL superstar Shawne Merriman tells TMZ Sports ... he was approached to be a part of Will Smith's "Concussion" movie but SHOT IT DOWN.
The former San Diego Chargers linebacker says he's convinced the movie is "definitely going to give the NFL a black eye" ... but says he didn't feel right jumping aboard the project.
The movie -- set to come out in December -- is supposed to blow the lid of an alleged NFL conspiracy to hide the debilitating life-long effects of concussions from players.
"I turned it down and said 'No' ... I don't want to be a part of it because I didn't know how they were going to make it look."
Still, Merriman says concussions are a problem among NFL vets -- saying, "A lot of people have problems and the NFL should fix it .. period."