Stabler, who died of colon cancer at 69 in July, had Stage 3 chronic traumatic encephalopathy, Dr. Ann McKee told The Associated Press. McKee said the disease was widespread throughout his brain, with "quite severe" damage to the regions involving learning, memory and regulation of emotion.
The diagnosis was first reported by The New York Times. The paper later reported that Earl Morrall, a two-time Super Bowl quaterback, was diagnosed with Stage 4 CTE. Morrall died in 2014.
"We've now found CTE in former NFL players who played every position except kicker," said McKee, a professor of neurology at Boston University. "While we know on average that certain positions experience more repetitive head impacts and are more likely at greater risk for CTE, no position is immune."
The disease, which can only be diagnosed after death, is believed to be the result of repeated brain trauma. The relationship between concussions and brain degeneration is not fully clear and researchers have cautioned that other factors could be contributors to symptoms that later have resulted in CTE.
In 2012, the NFL donated $30 million in funding for medical research to the Foundation for the National Institutes of Health. The unrestricted gift was the NFL's single-largest donation to any organization in the league's 92-year history and is overseen by NIH.The left-handed Stabler, nicknamed "Snake" for his ability to escape from defenders, led Alabama to an undefeated season in 1966. A second-round draft pick by Oakland, he was the NFL's Most Valuable Player in 1974, and led the Raiders to victory in the 1977 Super Bowl.
The Associated Press contributed to this report.