In a stunner, 49ers linebacker Chris Borland retires after one season in NFL

BERKELEY, Calif. -- In a stunning development, San Francisco 49ers linebacker Chris Borland announced Monday he is retiring after just one season in the NFL because of concerns about head trauma, ESPN reported.

Considered a rising star in the NFL, Borland, 24, was expected to be a key part of the 49ers defense this season after the retirement last week of All-Pro linebacker Patrick Willis.

Borland replaced Willis, 30, when Willis sustained a toe injury -- and impressed coaches and fans with his play.

Borland told ESPN's "Outside the Lines" Monday that he notified the 49ers of his decision Friday, after consulting with family, friends, former teammates and concussion researchers.

The third-round draft pick from the University of Wisconsin said he has had just two diagnosed concussions -- one while playing soccer in the eighth grade, and the other playing football as a sophomore in high school.

But he has been concerned about the degenerative effects of head trauma.

"I just honestly want to do what's best for my health," Borland told ESPN. "From what I've researched and what I've experienced, I don't think it's worth the risk."