Cast member Will Smith poses during the premiere of the film "Concussion" during AFI Fest 2015 in Hollywood, California, November 10, 2015. REUTERS/Kevork Djansezian

Hollywood star Will Smith deals with football-related brain trauma in “Concussion”, a sports drama which has already drawn some controversy ahead of its release.

The film tells the story of Nigerian-born forensic pathologist Dr Bennet Omalu, who diagnosed Chronic Traumatic Encephalopathy (CTE), a degenerative brain disease, in U.S. football players.

In September, the New York Times reported Sony Pictures Entertainment executives had altered the movie’s script to avoid antagonizing the National Football League (NFL). It said marketing plans were positioned to focus on the story of a whistle-blower rather than a condemnation of the sport.

Sony at the time said the newspaper’s story had contained “many misleading references” and that nothing had been “softened” in the movie to placate anyone. The NFL declined to comment on the report but said it was encouraged by the focus on player health and safety.

“Men in Black” actor Smith, who plays Omalu, premiered the movie at the American Film Institute’s AFI Fest on Tuesday.

“It was personal for me from day one, when I sat down with Dr Omalu and … he said to me, when he was a boy growing up in Nigeria all he ever wanted to be was an American,” Smith said.

“The thing that was a revelation for me with this is that is not really the big hits that are the problem, it’s definitely an issue, but more the issue is the repetitive head trauma.”

The NFL in April settled a lawsuit brought by about 5,000 former players who accused it of covering up the dangers of concussions.

Former football player Leonard Marshall, who suffers from CTE-related illnesses, said the film serves as an educational tool.

“It means a lot to me to see the word get out and the science be exposed behind Chronic Traumatic Encephalopathy,” he said.

“Concussion” opens in U.S. cinemas on Dec. 25 and early 2016 elsewhere.