A.J. Tarpley remembers having no reaction. None. When San Francisco 49ers linebacker Chris Borland surprised the rest of us by retiring in early 2015 after his rookie season, Tarpley was a similar, scrappy linebacker clawing his way toward the first of what he trusted would be several NFL seasons.

Tarpley was too focused, too driven to relate to Borland — who retired at 24 because of concerns about concussions — or any other players who were bucking NFL tradition and leaving the game they love before it exhausted their youth or health.

“I think I was indifferent,” said Tarpley, a Plymouth native who overachieved from Wayzata High to Stanford to the Buffalo Bills. “At the time Borland retired, I had one concussion in high school and one concussion in four years at Stanford.

“I knew that all the research surrounding concussions and the brain were starting to pick up. I respect people in whatever they decide to do. But I never thought I’d ever come to a similar decision. Never. Never in my life.”

But he did. After two concussions last season, he retired in April at the age of 23.

Tarpley loved football. Still does. He started playing in third grade when his coach/father let him practice with his older brother’s fourth-grade team. By seventh grade, A.J. was dieting to make junior high weight limits. By the time he left Stanford for Buffalo as a rookie free agent, Tarpley had “played through fractured bones, sprained ligaments, muscle strains, bone spurs and shoulder dislocations” while never missing a practice or a game in 15 years, as detailed in an article he wrote for Sports Illustrated’s “Monday Morning Quarterback” website.

But perhaps Tarpley is the NFL’s new normal. Not an exodus epidemic, but a steady drip of young players who no longer can ignore the long-term cognitive risks in light of research connecting the degenerative brain disease chronic traumatic encephalopathy (CTE) and former NFL players who suffered greatly before disease or suicide ended their lives.

Besides Borland, other players 30 or younger to retire after the 2014 season included Anthony Davis, Jason Worilds, Jake Locker and Patrick Willis. Players 30 or under joining Tarpley in early retirement this offseason include B.J. Raji, Jarod Mayo, former Vikings safety Husain Abdullah and, of course, Calvin Johnson.

“It’s an interesting scenario,” Tarpley said. “I think guys will walk away on their own terms more. I think the bigger impact than a guy like me is seeing guys like Calvin Johnson and Marshawn Lynch and guys who can obviously still play football walk away.

“They’ve already made their generational wealth. They’ve gotten their share of accolades, achievements and goals. And they’re satisfied and walking away early.”

Former Vikings receiver Sidney Rice retired in July 2014 while with Seattle. He was 27, suffered multiple concussions and admitted to CBS News that he was frightened about ending up like former players who have experienced cognitive decay later in life.

“I wanted to be able to function,” Rice said.

That’s essentially the reason Abdullah used when he retired after seven NFL seasons. He turned to Instagram to thank the Vikings for giving him an opportunity as an undrafted rookie in 2008, and the Kansas City Chiefs for giving him three years that “may have been the most enjoyable football experience in my entire life.”

“There are numerous deciding factors in my decision, with personal health being foremost,” Abdullah wrote. “Sitting for five weeks last year after suffering the fifth concussion of my career, I had a lot to contemplate. My goals moving forward are to be of benefit to my family, my community, my country and hopefully the world. Having a sound mind will be vital in accomplishing these goals.”

Late last month, Bills General Manager Doug Whaley made headlines when asked whether the team’s top receiver, Sammy Watkins, 22, is injury prone.

“This is the game of football,” Whaley told WGR 550 radio. “Injuries are part of it. It’s a violent game that I personally don’t think humans are supposed to play.”

Whaley was trying to protect Watkins, whom Whaley selected fourth overall in 2014 after surrendering the ninth overall pick that year, and picks in the first and fourth rounds in 2015. But the comment — which he later tried to clarify with a statement saying he used a “poor choice of words” — spoke to the much broader issue of player safety at a time when the league is struggling with its image in that regard.

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Tarpley’s first concussion was mild, he said. It came during his junior year at Wayzata and he didn’t miss a game.

The second one came when he was a junior at Stanford. It wasn’t so mild.

“That was probably the hardest hit — a teammate flying in and accidentally hitting me in the side of my helmet with the crown of his,” Tarpley said. “That one had the most memory loss. It was the Pac-12 Championship, so we had the whole Rose Bowl ceremony bid afterward. I have no recollection of being out there at the ceremony. Nothing.”

Tarpley left the field after the hit, went back in for a play and returned to the sideline.

“What people have told me is I asked one of my teammates who had been out weeks why he wasn’t dressed for the game,” Tarpley said. “That’s when they knew something was wrong.”

Tarpley’s third concussion came last summer in training camp with the Bills. Not good timing for an undrafted rookie. So Tarpley hid his symptoms.

“I had migraine headaches days afterward,” Tarpley said. “But, first and foremost, all the guys who make it to the NFL will do anything to get there. I don’t regret my mentality or anything about it. You’ll do anything to get there.”

The 6-2, 241-pounder made the team as a special-teamer and backup linebacker. Unfortunately, his fourth concussion was fast approaching. While playing his first defensive snap against the Jaguars on Oct. 25 in London, Tarpley was lowering himself to tackle a running back when the crown of the running back’s helmet struck the side of Tarpley’s helmet. His vision blurred and closed until he said it was like looking through horse blinders.

But Tarpley stayed in the game. He blew an assignment on the next snap and got pulled but said nothing of his concussion symptoms.

When Buffalo’s goal-line defensive package was called in before the next play, Tarpley buckled his chinstrap and went back onto the field.

“There was no way I was going to let the guys down at that point,” Tarpley said.

The Bills stopped the Jaguars on all four downs. Tarpley left the field and finally pulled himself from the game.

“We had a bye the next week, so I was lucky,” Tarpley said. “I didn’t feel right for two weeks afterward. But even if that meant lying about your symptoms, it was getting better, so I lied about feeling completely back to normal. I kind of held on to the fact that, ‘OK, I don’t feel right, but in two days, I’m hoping I will.’ ”

Tarpley played in the next four games, got released, spent a week on the practice squad and was re-signed for the final four games. He had an interception in each of his last two games. His game-clinching interception in the season finale against the Jets was the final play of his NFL career.

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Tarpley said he feels great. He has no symptoms and he just turned 24. Football season is around the corner, and sitting there on the table in front of him was a contract worth about $500,000 for 2016 and a Bills team that wanted him after not re-signing starter Nigel Bradham or backup Ty Powell this offseason.

Yet Tarpley says he is at peace because he spent three months in deep introspection, purposely keeping it from friends and family to protect them should he decide to keep playing and suffer a debilitating concussion down the line. He searched online for unbiased information and used the knowledge he gained while in the NFL’s concussion protocol.

“Could I block out everything I had learned and what I had essentially guessed my risk was for the future?” Tarpley said. “If you can’t do that, you can’t play this game. One of the toughest things I’ve had to do was go and tell Coach [Rex] Ryan, who gave me my shot, that I was retiring.”

Tarpley said he “has nothing but respect for Coach Ryan and the entire Bills organization.” Through it all, Tarpley said they had “nothing but my health in mind when they were treating me.” It was Tarpley whom Tarpley could no longer trust.

“It’s tough,” he said. “I think a lot of people are surprised by how much I still love football. They see the headline, ‘23-year-old retires’ and assume I didn’t love the game. I do and I also make sure to point out in every interview and whatever words I use in my article that I’m not a victim here. I don’t think football is a bad thing.

“I know people have their agenda and will use anything they can to encourage their cause. But I want to be void of that. There’s no political agenda here.”

In fact, Tarpley said his decision very well could have gone the other way under different circumstances.

“If I had four concussions and I had only one more year until my second contract, a player’s big-money contract, where you build your family’s generational wealth, I think I would have thought differently,” Tarpley said. “Or at least it would have played into my decision.”

Ryan said he respected Tarpley’s decision when reporters asked about it in April.

“Everybody has to make decisions, whether you play, whether you don’t play, and things like that,” he said. “I’ll say this, I certainly respect A.J.’s decision to move on. But again, everybody makes decisions of what they feel is in their best interests, and I can tell you that’s what A.J. thought.”

But, ultimately, Tarpley decided that playing linebacker meant continual situations that would be exactly like the ones that caused his concussions. So he has moved on to Plan B options, which include using his science, technology and society degree and/or going into investment banking, private equity real estate or maybe even coaching.

“Maybe if I had just suffered freak hits,” Tarpley said. “Or maybe if I had just been caught in the wrong scenario, I would have looked at all this differently. But those were hits that would need to happen.

“I knew I would be in those exact scenarios again. And what was I going to do in that scenario? Out of respect for my health and the sport, I had to retire.”