You can still count on two hands how many players walked away from a pro football career with more good days likely ahead of them than behind.
That's why Chris Borland's decision to retire at age 24, after a great rookie season with the 49ers and a near-guaranteed huge payday down the line, sent shockwaves rippling across the NFL. The announcement came within days of retirements by 20-somethings Jason Worilds, Jake Locker, Cortland Finnegan and 30-year-old perennial Pro Bowl 49ers linebacker Patrick Willis, whose spot Borland was expected to fill.
Each of those players cited different reasons for calling it quits. Only Borland, who suffered a concussion in training camp last fall but covered it up in a bid to win a spot on the field, tied his departure directly to the continuing risk of brain injury.
"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?'" Borland said Monday on ESPN's "Outside the Lines.''
But more surprising, and perhaps even more unsettling for the league's long-term prospects, was the reaction from fellow players and the game's notoriously tough fan base: an almost-unanimous show of respect for a young player who left millions on the table rather than expose himself to more concussions.
"I didn't see it coming,'' Chris Nowinski, an expert on sports-related concussions, said about Borland's announcement. "This was somebody who got educated on the issue, and the choices he was facing. I wasn't sure if there were current players interested enough to do their homework.''
But a moment later, Nowinski, a Harvard graduate, author and former pro wrestler who was instrumental in the formation of Boston University's Center for the Study of Traumatic Encephalopathy, said the reaction from the larger NFL community may have been more surprising still.
"It shows the macho culture of `destroy yourself for the game' is losing its grip, that it's no longer cool to question people's toughness. That represents a big shift in thinking from just a few years ago,'' he said. "The real question now is whether those players who stay in the game will be encouraged to become greater advocates for their own safety. ...
"And then there's the 3 million or so youngsters playing the game at the lower levels,'' Nowinski added. "Will something like this lead to better and more effective safety measures for them?''
The depth of that talent pool prompted a response from Eliot Wolf, director of player personnel from the Green Bay Packers, who tweeted: "Anyone worried about the future of football should see the amount of calls and emails we get from kids literally begging to get into pro days.''
But his reaction, as well as a statement from the NFL repeating the claim that "football has never been safer,'' was largely washed over by a wave of support on social media from Borland's former teammates at Wisconsin and San Francisco, as well as NFL opponents he bruised en route to a host of rookie honors and a team-leading 108 tackles. Typical was this tweet from St. Louis Rams guard Chris Long: "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.''
Nowinski believes much of the credit for the paradigm shift in how players, NFL fans and even the league itself views concussions is an outgrowth of the continuing research at Boston University's CTE center. Because CTE -- a degenerative disease which often results in memory loss, aggression, confusion and depression -- can only be diagnosed during an autopsy, he helped convince the families of several deceased players to donate their brains. Dave Duerson, who committed suicide by shooting himself in the chest, made his wishes to be part of the study, in a note he left behind.
The CTE center has confirmed that 76 of the 79 NFL players whose brains were examined showed signs of degenerative brain trauma.
"Borland's decision won't lead to a rush of players out the door,'' Nowinski said. "But it should point in the direction of making sure that those who stay get the safest environment possible, and that we have that discussion without pretending the dangers don't exist. ... They do. Our job now is to build on that, especially for those kids too young to make a risk assessment for themselves.''
Meanwhile, Santa Clara University's Institute of Sports Law and ethics suggests the league may increasingly become a place for those with other options.
"It's likely we'll find that young men who stick with football over a long period of time tend to be from less fortunate economic backgrounds," said Mike Gilleran of the Institute of Sports Law and Ethics.
NBC Bay Area's Nannette Miranda contributed to this report.
