"It was a complete and total shock to me," Hernandez's lawyer said of his death.
Such damage has been seen only in people over 46. He was 27.
Hernandez ended his life in prison in April.
Boston University researchers will study the former New England Patriots player's brain for signs of chronic traumatic encephalopathy, or CTE.
Players had fought the deal because it doesn't cover future victims of brain disease CTE.
NFL officials denied the link as recently as February.
Heath professionals have written an open letter calling for a switch to touch rugby for young people.
The film, which opened on Christmas Day in the U.S., examines the brain-injury crisis.
The question isn't, "Is football dangerous?" It's, "Can it ever be safe?"