SAN FRANCISCO — Another star member of San Francisco's vaunted defense is calling it a career.

The 49ers announced late Monday that linebacker Chris Borland is retiring after one season, without offering specifics.

Borland told ESPN's "Outside the Lines" earlier Monday that he is retiring following his standout rookie season because of concerns about head trauma.

"While unexpected, we certainly respect Chris' decision," 49ers general manager Trent Baalke said. "From speaking with Chris, it was evident that he had put a great deal of thought into this decision. He was a consummate professional from day one and a very well-respected member of our team and community.

"Chris is a determined young man that overcame long odds in his journey to the NFL and we are confident he will use the same approach to become very successful in his future endeavors. We will always consider him a 49er and wish him all the best."

The 24-year-old Borland had a team-leading 108 tackles in his stellar rookie season, emerging as a punishing defender. He also had a sack and two interceptions.

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He told "Outside the Lines" on Monday he wants to do "what's best for my health."

"From what I've researched and what I've experienced, I don't think it's worth the risk," he said in that interview. "I feel largely the same, as sharp as I've ever been. For me, it's wanting to be proactive," Borland said. "I'm concerned that if you wait till you have symptoms, it's too late."

This is yet another blow to the team that lost coach Jim Harbaugh and defensive coordinator Vic Fangio after the season, then watched Frank Gore, Mike Iupati, Chris Culliver and Perrish Cox depart in free agency this past week. Defensive line coach Jim Tomsula was promoted to head coach following an 8-8 season and the franchise's first time out of the playoffs in four years.

Borland's big announcement comes less than a week after five-time All Pro linebacker Patrick Willis walked away from football.

In a strange twist, Borland is the one who replaced Willis in the starting lineup after his October toe injury that required surgery.

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The NFC Defensive Rookie of the Month for November, Borland injured an ankle on the final play of the first half against the Seahawks on Dec. 14 and didn't play again last season.

The 49ers selected Borland in the third round of the draft out of Wisconsin, where he was an imposing pass-rusher.