Duper, who was a star with the Miami Dolphins for 11 seasons in the 1980s, told ESPN he was feeling the effects of cognitive dysfunction and had been diagnosed just this week with chronic traumatic encephalopathy.
"It was shocking," Duper told ESPN's "Outside the Lines" Friday. "I hoped nothing was wrong."
But something was wrong, ESPN.com said.
Duper admitted to bouts of depression and forgetfulness, and damage to his brain was confirmed by advanced medial scans.
"I have had memory things where I would go to the store and forget what I went for," he said. "And I have emotional swings and panic attacks."
Duper is the ninth living NFL veteran to be diagnosed with CTE, ESPN.com said. In addition, autopsies on more than 50 former players found buildups of a protein that causes CTE in their brains.
Duper said he was holding his own, and hoped his diagnosis would build further momentum for large-scale testing of NFL players.
"I think this test will show the damage we are doing to our bodies and how to prevent it," he said. "People have to speak up about CTE. I hope me speaking out will show that people should be tested."