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 fans: 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?"

On social media Borland drew waves of support from ex-teammates and foes.