IF YOU ever want to dabble in time travel, try whispering two words to Philadelphia Eagles fans of a certain age: Andre Waters.

Let it sink in for a moment, like a secret password, and watch the years disappear from their faces. Wistful smiles replace the usual heavy frowns.

Waters was an underdog who had to fight and claw for his job - not the most naturally gifted guy on the field, but one of the toughest.

He was, in other words, the embodiment of all the blue collar traits Philly sports fans always hope to see in a local athlete. And to top it off, he was that guy on the beloved Buddy Ryan teams of the late 1980s and early 1990s.

You could spend all day talking about Reggie White's towering Hall of Fame talent, or Randall Cunningham's otherworldly athleticism.

Waters brought sheer ferocity to the table. He was a human heat-seeking missile in the Eagles' secondary, capable of obliterating any receiver or running back who had the misfortune of carrying the ball near his field of vision. The fans in the stands loved him for it. Buddy did, too.

But those explosive hits - remember the "House of Pain" game? - carried a price. Waters took his life at the age of 44 in November 2006, desperate to escape the unbearable torment of dementia and depression.

His friends and family were dealt a second blow a few months later, when neuropathologist Bennet Omalu determined Waters' brain had been ravaged by chronic traumatic encephalopathy (CTE), a degenerative disease linked to repetitive head trauma.

Bone-rattling tackles and God knows how many concussions had left Waters with the mind of an 85-year-old Alzheimer's patient.

Waters' tragic story is going to be revisited in the upcoming Will Smith movie "Concussion." Smith stars as Omalu in the film, which opens on Christmas Day and explores the Nigerian-born doctor's efforts to raise awareness about CTE in the face of fierce opposition from the NFL.

The Daily News recently reached out to a handful of Waters' old teammates and friends to discuss the movie. But most didn't know that their friend's grim fate was part of the film.

It was the kind of news that could rip open an old wound, allowing a river of mixed emotions to seep out from below. Time is supposed to heal stuff like that, but it never does, not really.

Eric Allen was just 22 when he met Waters for the first time.

The Eagles had made Allen, a cornerback with great hands and even better instincts, their second-round draft pick in the spring of 1988, and decided to have him room with Waters at training camp that summer.

"He was my mentor. He was really responsible for showing me how to go about conducting myself on the road," said Allen, who retired from the NFL in 2001 and now works as an analyst for ESPN.

"His back story was really unique, and he would tell it all the time."

Waters grew up in abject poverty in rural Florida, the ninth of 11 children.

His path to the NFL didn't include a full-ride scholarship to some powerhouse college football program. Waters attended Cheyney University and landed on the Eagles as an undrafted free agent in 1984.

At 5-foot-11, he wasn't much more than a "spare part" on special teams, according to former Daily News sports columnist Ray Didinger.

But then Buddy came to town in 1986, full of his trademark swagger, the architect of the legendary defense that helped lead the Chicago Bears to a Super Bowl title.

"When Buddy began to rebuild the defense here, he wanted guys who were aggressive and hard hitters, and Andre definitely was that kind of player," Didinger said. "He was the perfect strong safety in Buddy's defense."

But it wasn't enough to just be good at knocking the hell out of a guy on the other team.

"A lot of people overlook how smart Andre was," said former Eagles middle linebacker Byron Evans, who was drafted by the team in 1987.

"He studied the game. He knew what the other team was going to do."

Evans paused. He and Waters developed a close bond that lasted well beyond their playing days.

"You know, I wouldn't have made it as far as I did if it hadn't been for Andre. When I had trouble with the '46' defense, he helped me," he said.

"He helped me to study and took me under his wing. He gave me a friend to lean on."

Didinger recalled watching Waters go out of his way during training camp to spend time with fans.

"It would be 100 degrees, guys are sweating, and I'd see Andre go over to where the fans were," he said. "He'd walk down the line, sign everything, and get down on his knees and talk to a kid in a wheelchair. They were genuine acts of kindness."

On the field, there was no trace of the kind, compassionate guy that Waters' friends still recall warmly.

In the run-up to an Eagles-Dolphins game in 1990, an article in the Sun-Sentinel nervously recounted Waters' reputation for delivering devastating - and in some cases, career-ending - hits.

He couldn't outrun that notoriety, or the nickname that came with it: "Dirty Waters."

It was a double-edged sword. The Eagles wanted other teams to be scared of facing them.

"People understood that it was going to be a long day when you played us," Allen said during a recent phone interview.

"We took pride in that aspect of the game - being smart, being tough, being a tackling football team. We intimidated people, but we were also respected."

Privately, Waters loathed his nickname.

"It bothered me that when you mentioned his name around other people, the first thing they'd say was, 'Oh yeah, he's a dirty player,'" Didinger said.

"Objectively, yeah, some of the hits were over the line. But I think there was a heck of a lot more good to Andre Waters than people were willing to give him credit for."

Few people seemed to understand that Waters wasn't walking away from big hits unscathed. He once told the Inquirer that he stopped counting the number of concussions he'd sustained at 15.

"You had to go back in the game. You couldn't tell Buddy Ryan you weren't going back in the game, and you couldn't tell your teammates you weren't going back in," Evans said.

"That's what you were taught. We played to the max. We didn't know the effects."

Players who might have had concussions made their way back to the huddle, where they would describe "seeing stars" or complain that they'd had "their bell rung," Allen said.

"It was almost an automatic call to keep that guy on the football field," he said, "but you try and protect the defense by running a Cover 2 or a Cover 3 and put them in the middle of the field until they came back to the huddle and could resort back to the normal process."

Sometimes, Allen said, teammates would help a dazed player line up correctly before the start of the next play.

"In that time, that's how you went about playing the game," he said.

Waters led the Eagles in tackles four times, including a career-high 156 in 1991. You look back and wonder: how many of those tackles led to concussions that were untreated? How many came on plays where Waters shouldn't have even been on the field?

Questions are all Waters' loved ones are left with after all these years, and none of them have any good answers.

When his playing career ended in 1995, Waters desperately wanted a second act as a coach in the NFL.

He found jobs at small universities.

"He was always busy, always working," said Jerrold Colton, Waters' lawyer and longtime friend.

"He didn't struggle as much as a lot of guys do with the transition from playing. He was always down-to-earth. As much as he loved being an NFL player, it never consumed him."

But Waters never landed a coaching job in the NFL. He worried that the "Dirty Waters" reputation had worked against him. And with Ryan retired by then as well, there weren't any coaches around to offer him a break.

Little by little, he also began to experience problems with his memory.

"Seth Joyner and I saw Andre a couple of years before [his death], and he was mentioning to us that he was having trouble remembering things," Allen said.

"He would go to some of the coaching clinics the league held, and people were a little bit surprised by him not being able to get on the board and show some of the things he had just been talking about."

Like anyone who has lost a friend to suicide, Allen mulls over the what-ifs.

"You always think about the opportunities you may have missed, the signs you may have missed, to offer support and help."

Colton was close to Waters, but didn't know about his memory lapses. In their conversations, Waters' complaints centered on aches and pains, his frustration about not being able to land a job in the NFL, and not seeing his three children as often as he wanted.

"Andre was wrestling with certain things, but I never linked any of it to concussions," Colton said. "There wasn't all of the information out there that we have today. I wish I'd been more informed."

On Nov. 20, 2006, Waters shot himself with a .32-caliber Smith & Wesson handgun at his home in Tampa, Fla.

Former professional wrestler Chris Nowinski persuaded Waters' sister to allow Omalu to study her brother's brain tissue.

Nowinski's career was ended by concussions. He co-founded the Concussion Legacy Foundation, and remains a leading voice on the issue.

"Football killed him," Omalu matter-of-factly told the Palm Beach Post in a 2010 story about Waters.

He wasn't the first ex-NFL player with CTE to commit suicide - or the last.

In May, Adrian Robinson Jr. - a Temple University product who played for the Pittsburgh Steelers and Denver Broncos - committed suicide in Philadelphia. Doctors found CTE in his brain.

Robinson was just 25.

Current and former NFL players are conflicted about this topic being explored in a Hollywood film.

"I think, ultimately, a movie is made to make money," Allen said.

"I have mixed feelings, of course, because I really love the game of football. I have four sons, and they all play."

Allen stressed the importance of teaching kids how to tackle and practice in a way that reduces the chance of injury.

During a recent appearance on WMMR's "The Preston & Steve Show," Eagles tight end Zach Ertz, who missed several games this season due to a brutal concussion, said he wouldn't see "Concussion."

Both Allen and Evans expressed concern about how the movie will portray Waters.

A review on Time magazine's website noted that a scene showing Waters begging for help at the NFL's headquarters didn't actually happen in real life.

When a reporter told Evans that Waters was going to be a character in the movie, his first concern was how Waters' family would feel about it. (They could not be reached for this story.)

"I still speak to his mom. That's how I cope," he said. "He was one the most genuine people I've known in my lifetime. Very loving."

Evans isn't opposed to the public getting a good look at the health problems that former players contend with.

"Look, it needs to be addressed, probably now more than ever," he said. "Sometimes I forget things."

You can still see Waters sometimes, in YouTube clips - or on your father's old VHS tapes - of the Gang Green defense mauling opponents in Veterans Stadium.

He's a live wire, fast and strong, seemingly indestructible. And he was. Until he wasn't.

"He was the prototypical Eagles player," Allen said.

"He came from a difficult situation. He wasn't a prince riding in on a white horse. But he was willing to give everything to the city and the team. That's what Andre was."

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