"It hit me like a ton of bricks," said Dorsett of his diagnosis with chronic traumatic encephalopathy, the Dallas Morning News reported Thursday.
ESPN had earlier reported Dorsett and two other former NFL players, Hall of Fame offensive lineman Joe DeLamieulleure and former All-Pro defensive lineman Leonard Marshall, had been diagnosed with the condition after three months of testing at UCLA.
The university tested five other former athletes last year and found signs of CTE, the first time researchers had found the disease in living players.
CTE is characterized by a buildup of an abnormal protein called tau that kills brain cells.
It has no known cure, but Dorsett said researchers told him it can be slowed down and even reversed.
Dorsett said he received the news Monday from a researcher at UCLA.
"I'm being proactive," he said. "I'm trying to cut it off at the pass, slow it down, do whatever I can to fight this thing. But it's tough, man, it's frustrating as hell at times."
The Hall of Famer has battled memory loss and depression for years.