One of this year’s most promising NFL rookies said he would retire this week over concerns about traumatic brain injuries from playing the game, ESPN reported.

Chris Borland, a top linebacker for the San Francisco 49ers, said that he’s in good health now, but has concerns about his future.

“I feel largely the same, as sharp as I’ve ever been. For me, it’s wanting to be proactive,” Borland told Mark Fainaru-Wada and Steve Fainaru for ESPN’s Outside the Lines. “I’m concerned that if you wait ’til you have symptoms, it’s too late. … There are a lot of unknowns. I can’t claim that X will happen. I just want to live a long, healthy life, and I don’t want to have any neurological diseases or die younger than I would otherwise.”

Borland’s departure is a blow to the NFL, which has been determined to show that it is taking seriously growing concerns about the long-term effects of concussions and working to make the game safer. The news also comes as the NFL is embroiled in a legal battle with thousands of former players who have sued the league for compensation for head injuries and amid new evidence that even high school players can develop chronic traumatic encephalopathy, or CTE, a debilitating brain disease linked to head trauma.

FRONTLINE reported on the devastating effect of repeated head injuries in football in League of Denial, which found that not only did the NFL know about the risks of concussions years earlier, it backed away from its own research on the issue.

Borland is walking away from a half-million dollars this season alone. But the star player also told ESPN that he has already been diagnosed with two concussions playing sports when he was still in elementary and high school. And, he said he was troubled by the stories of veteran NFL players such as longtime Pittsburgh Steelers center Mike Webster; Dave Duerson, a former defensive back for the Bears; and Ray Easterling, a retired Atlanta Falcons safety, all of whom were diagnosed with CTE after their deaths. Easterling and Duerson both took their own lives.

“You read all these stories, and to be the type of player I want to be in football, I think I’d have to take on some risks that, as a person, I don’t want to take on,” Borland said.

Borland said he made his decision after consulting concussion experts, family, friends and former players.

“From what I’ve research and what I’ve experienced, I don’t think it’s worth the risk.”

Related Film: League of Denial: The NFL’s Concussion Crisis

Watch FRONTLINE’s investigation into what the NFL knew about the risks of head injuries and when it knew it.

Sarah Childress, Former Series Senior Editor, FRONTLINE