Borland was a rookie linebacker in his first training camp in August when he got his "bell rung," he told ESPN's "Outside The Lines."
What did he do? He kept playing that day, and through an excellent rookie season. But the thought of the permanent damage that might be happening to him kept nagging at him.
At age 24, not even yet in his prime as an NFL player, Borland told his team he was retiring because he was worried about the long-term effects of head trauma.
"I just honestly want to do what's best for my health," he told ESPN. "From what I've researched and what I've experienced, I don't think it's worth the risk. ... I'm concerned that if you wait (until) you have symptoms, it's too late."
It was after that self-described "minor concussion" in the preseason that Borland threw himself into researching what had happened to other football players. In the end he didn't want to leave his teammates in a bind, but, "I know this is right for me."
He said teammates' reactions to his retirement were mixed. One of his best friends told him that he was crazy to walk away from the money. Borland, a third-round pick from Wisconsin, had signed a nearly $3 million contract for four seasons with the San Francisco 49ers and banked a $600,000 bonus.
The guys who wanted him to come back would say to the 49ers top tackler, "That's a lot of money. Why don't you get your money and get out," Borland recounted to ESPN.
The thought made him uncomfortable because he doesn't want to do anything just for a big paycheck.
He didn't want to be wrong when it comes to this: "Who knows how many hits are too many?"
Part of a larger discussion
Borland is one of a handful of players to retire young but possibly the first to retire before concussions became an issue for him. He's also now part of a shift in thinking about what repeated head injuries can do to an athlete. Reports show an increasing number of retired NFL players who have suffered concussions developed memory and cognitive issues such as dementia, Alzheimer's disease, depression and chronic traumatic encephalopathy, or CTE.
"For me, it's wanting to be proactive," Borland said. "I'm concerned that if you wait (until) 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."
Passionate reactions to his decision came quickly, with many supporting him.
Others say that while the move might be smart for Borland, and even encourage other players to speak out and stand up for their health, there will always be plenty of guys eager to replace players like him.
Borland told ESPN that he wasn't saying no one should play football. He said youth players and their parents should do things: 1) Get informed about concussions and 2) Never let anyone play
'Risks ... I don't want to take on'
Last August, thousands of former NFL players and their families reached a deal in a class-action suit that called for the NFL to cover the cost of concussion-related compensation, medical exams and medical research for retired players and their families.
The suit alleged that the NFL deliberately misled players about scientific data that the medical community had found about the risks associated with concussions. In July 2014, a federal judge granted preliminary approval to the landmark deal but she has yet to give final approval to the settlement.
Chris Dronett was one of the plaintiffs. Her husband, former Denver Bronco Shane Dronett, committed suicide in 2009 when he was 38. After his death, scientists found evidence of CTE in his brain.
Borland named three players he said made him rethink a life in the NFL.
"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," he said.
Webster had a career with the Pittsburgh Steelers and Kansas City Chiefs from 1974 to 1990. He was the first former NFL player to be diagnosed with CTE. After he retired, he was diagnosed with amnesia, dementia, depression, and bone and muscle pain. He was 50 when he died.
Duerson killed himself with a gunshot to the chest. He had sent a text to his family asking that his brain be sent to Boston University School of Medicine, which was researching CTE. BU neurologists confirmed the NFL veteran had the disease.
Easterling, who played eight seasons with the Atlanta Falcons, committed suicide in 2012. He apparently suffered dementia. An autopsy revealed he had CTE.
Some on social media said Borland's decision made them think about former NFL linebacker Junior Seau. He was 43 when he was found dead with a gunshot wound to the chest. Friends and family members say multiple concussions were to blame for the suicide, but an initial autopsy report found no apparent brain damage.
Portions of Seau's brain were sent to the National Institutes of Health, which found "abnormalities ... that are consistent with a form" of CTE
He will be inducted into the Pro Football Hall of Fame this summer.
The NFL has reported that in 2013, 228 concussions were diagnosed from practices and games. At least 261 were diagnosed the previous year, the league said.
In its statement about Borland's retirement, Jeff Miller, the NFL's senior vice president for health and safety, said: "We respect Chris Borland's decision and wish him all the best. Playing any sport is a personal decision. By any measure, football has never been safer and we continue to make progress with rule changes, safer tackling techniques at all levels of football, and better equipment, protocols and medical care for players."
Miller added that the league understands there is more work to do to improve player safety.
Complaints keep coming. In July of last year, ex-NFL players Christian Ballard and Gregory Westbrooks filed suit against the NFL Players Association, alleging the union withheld information about head injuries.