SANTA CLARA — Chris Borland, the 49ers’ leading tackler last season as a rookie replacement to Patrick Willis, is retiring because of concerns over head trauma, he told ESPN on Monday.

Borland’s stunning retirement is the latest blow this offseason to a 49ers team that has changed head coaches and lost six starters from last season.

“I just honestly want to do what’s best for my health,” Borland said on ESPN’s “Outside the Lines.” “From what I’ve researched and what I’ve experienced, I don’t think it’s worth the risk.”

Borland, 24, figured to be the heir apparent to Willis, who retired last week at age 30. Defensive tackle Justin Smith, a 14-year veteran, also is weighing retirement.

San Francisco 49ers’ Chris Borland (50) at a game against the Kansas City Chiefs in  in Santa Clara, Calif., on Oct. 5, 2014. (Dan Honda/Bay Area

“While unexpected, we certainly respect Chris’ decision,” general manager Trent Baalke said in a statement. “From speaking with Chris, it was evident that he had put a great deal of thought into this decision.

” … Chris is a determined young man that overcame long odds in his journey to the NFL,” Baalke added, “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.”

Borland told ESPN he notified the team of his decision Friday, after consulting with family members, concussion researchers, friends and current and former players.

Borland could not be reached for further comment. His agent, Neil Cornrich, did not immediately return a phone message.

Borland’s rookie season came to a painful end Dec. 14 at Seattle, when Seahawks quarterback Russell Wilson upended him on an interception return. Borland did not practice the following week with what the team listed as an ankle injury, and he was placed on season-ending injured reserve Dec. 20.

Borland told ESPN he has had just two diagnosed concussions — while playing soccer in the eighth grade and playing football his sophomore year in high school — but that he also experienced a potential concussion during 49ers training camp last summer.

Borland (5-foot-11, 248 pounds) recalled of that incident: “I just thought to myself, ‘What am I doing? Is this how I’m going to live my adult life, banging my head, especially with what I’ve learned and knew about the dangers?’ “

Gary Plummer, a former 49ers linebacker and broadcaster, is coping with post-career health issues, including what he termed the onset of dementia. His pro career spanned 15 seasons, starting with the USFL’s Oakland Invaders (1983-85) and ending with the 49ers (1994-97).

“Had I played in this particular era, with the knowledge players are now armed with, I know I wouldn’t have played 15 years,” Plummer, 55, said last week in a phone interview from his San Diego-area home. “Especially knowing what I know now.

“I have the early stages of dementia already, which sounds terrible, but you look statistically at all of us, once we get past our physical maturation, we start going downhill. The only difference is with the NFL, because of the concussions, it’s accelerated.”

Borland totaled 15 tackles in his first career start — an Oct. 19 loss at Denver — then piled up 22 tackles in a home loss to St. Louis and 21 tackles in a Nov. 9 win at New Orleans, according to statistics compiled by coaches’ film review. He finished with 126 tackles (in 14 games), according to the 49ers records; league game books credited him with 100 tackles.

Borland said to ESPN: “I’ve thought about what I could accomplish in football, but for me personally, when you read about Mike Webster and Dave Duerson and Ray Easterling, 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.”

Those former NFL stars were diagnosed with a devastating brain disease, chronic traumatic encephalopathy, or CTE, after their deaths. Duerson and Easterling committed suicide, as did former San Diego Chargers linebacker Junior Seau, who also had CTE.

Borland, according to ESPN, was having second thoughts about a prolonged career even before the end of the 49ers’ exhibition season. He informed his parents in a letter that he figured to have an abbreviated career.

Borland was a third-round draft pick out of Wisconsin. When Willis aggravated a toe injury in the second quarter of an Oct. 13 win at St. Louis — leading to Willis’ toe surgery and eventual retirement — Borland took over at inside linebacker and flourished as a starter over the next eight games.

What was once the NFL’s most dominant inside-linebacker corps is now its most uncertain. Aside from Borland and Willis retiring, NaVorro Bowman is attempting to come back from a knee injury that sidelined him all of last season. Also on the roster are Michael Wilhoite, Nick Moody and Shayne Skov.

Borland received $1,037,436 from the 49ers, including a $617,436 signing bonus. It’s unknown if the 49ers will attempt to recoup some of that signing-bonus money.

In the past week, the 49ers have lost Borland and Willis to retirement, as well as five of last season’s starters to free agency: running back Frank Gore, guard Mike Iupati, linebacker Dan Skuta and cornerbacks Chris Culliver and Perrish Cox. Wide receiver Michael Crabtree is not expected to re-sign with the club.

“It’s an incredible organization, and they truly looked out for players’ best interests,” Borland told ESPN about the 49ers.

He is the fourth player who has retired in the past week at age 30 or younger, including Willis (30), linebacker Jason Worilds (27) and quarterback Jake Locker (26).

Plummer was a friend to Seau, whose 2012 suicide brought attention to the brain trauma football players incur.

“Knowing now that’s what I would do to myself, I definitely would have retired earlier,” Plummer added. “This is some of the consequences that’s happened with the NFL concussion lawsuit. Guys now are armed with the knowledge they’re able to make an informed decision. We weren’t able to do that because we were lied to.”

Plummer remains very active in retirement, although instead of cycling 11,000 miles in a year as he used to do, he now enjoys gardening, doing yoga and learning to play the guitar.

“I’ve changed my focus,” Plummer said. “It’s about being physically fit. Having a fit body and an unfit mind is probably not the way I’d like to end it. There’s a ton of stuff you can do, and I’ll be proactive in it.”

As Baalke’s statement suggested, Borland’s retirement stunned many 49ers, as well as others around the league.

“I understand but still shocked,” cornerback Tramaine Brock tweeted.

The St. Louis Rams’ Chris Long, weighed in on Twitter: “WOW. I loved Chris Borland’s game but I can’t fault him for calling it quits. His concerns are real. Still it takes a man to do the logical.”

Wilson, who had that Dec. 14 hit on Borland after an Eric Reid interception just before halftime, offered Borland well wishes. Wilson, a 2011 college teammate, tweeted: “Pleasure playing with you Chris Borland (at Wisconsin) and against you for the 49ers. Praying for you.”

Seahawks linebacker Bobby Wagner reacted with a different take, tweeting: “No offense to anyone but I’m playing until I can’t anymore. I love this game to (sic) much.”

For more on the 49ers, see Cam Inman’s Hot Read blog at blogs.mercurynews.com/49ers.

Online extras

Mark Purdy: Recalling when a prominent figure in the NFL universe predicted such retirements.
Marcus Thompson II: Sacrifices such as Borland’s are required to produce real changes.
www.mercurynews.com/sports

Leading tacklers

Most tackles by a 49ers player during the 2014 season

1. Chris Borland 107
2. Michael Wilhoite 87
3. Antoine Bethea 86
4. Perrish Cox 53
5. Chris Culliver 45