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On Sept. 12, Floyd Mayweather Jr. (48-0), in his own estimation the greatest boxer ever, will step into the ring for what he insists will be the final time to face Andre Berto (30-3). Can Mayweather equal the great Rocky Marciano and end his Hall of Fame career at a staggering 49-0? Or will he be unsaddled on this final ride, finishing his career watching an opponent's hand raised for the first time as a professional?

The stakes, in theory, are high. But only in theory. Because no one, not even the most ardent Mayweather hater, believes Berto stands a chance, even against a disinterested 38-year-old version of the champ.

"Oh my god. I don't even want to break that fight down," former Mayweather foe Shane Mosley told ESNews. "I'm not even going to talk about that fight. It's not—that's, that's not a fight. I don't know if it's going to be worse than the Manny Pacquiao and Mayweather fight. I'm not sure. But I'm not talking about it."

The decision to fight Berto is perplexing at best, comical at worst. In fact, when Mayweather first floated the rumor, veteran ESPN boxing analyst Dan Rafael actually thought he was joking:

Mayweather, the most powerful fighter in the sport, the man whose box-office allure brought bitter rivals HBO and Showtime together, could have his pick of the litter. Fighters all but beg to step into the ring with him, happy not just with the opportunity to face greatness, but for the payday that comes with it.

The list of potential opponents is long. While no one could possibly match Pacquiao, some were fighters the audience might even have heard of.

So, why Berto?

It's a question that hangs over Mayweather and will continue to loom like a dark cloud over what should be a celebration, a victory lap commemorating a great career. After all, every time he's stepped up in class, Berto has fallen short. Now 31, he seemed destined to slink into obscurity, just another prospect who failed to live up to his potential.

By the Numbers: Andre Berto Height 5'6.5" Weight 147 pounds Age 31 Reach 68.5 inches Record 30-3 (23 KO) Rounds Fought 189 boxrec.com

Berto has no business being in the ring with Mayweather. But that's exactly where he'll be at Mayweather's home away from home, the MGM Grand Garden Arena in Las Vegas. It's a decision that's causing many in the boxing community to look back over Mayweather's entire career with a microscope, bringing up old concerns about his level of competition and propensity to choose his opponents carefully.

Ring magazine's Doug Fischer, a persistent Mayweather critic, laid out the case against Floyd's status as an all-time great, emphasizing the fights he didn't take rather than the ones he did:

I wanted to see Mayweather fight Joel Casamayor and Acelino Freitas at 130 pounds; Casamayor, Freitas, Stevie Johnston and Paul Spadafora at 135 pounds; Casamayor and Kostya Tszyu at 140 pounds; Antonio Margarito, Miguel Cotto, Vernon Forrest, Cory Spinks and Paul Williams at 147 pounds; and Forrest and Ricardo Mayorga at junior middleweight.

The truth is, you can make a list like that for almost anyone in the modern era of boxing. Every fighter has guys people would have liked to see them fight. The Balkanization of the sport and vast divides between rival promoters and competing television broadcasters often make certain fights all but impossible to put together.

If you wanted, you could pick Fischer's list apart fighter by fighter.

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Freitas, for example, made his name in America at the same time Mayweather was leaping up to lightweight for some of the biggest challenges of his career. Johnston was a fading force by the time his stablemate joined him at 135 pounds—in fact, he had failed twice against the man Mayweather challenged for gold in his first fight at lightweight. Tszyu, whose power would have surely challenged Mayweather, fought on Showtime when Floyd was exclusive to HBO.

You get the picture. The strongest case against Mayweather comes in the form of Cotto, whom he eventually defeated, and Margarito, who disgraced himself in a cheating scandal. With the privilege of hindsight, it's not particularly compelling.

Mayweather's record, in many ways, doesn't look much different than those of his predecessors or contemporaries. The greats of every era were criticized for their level of competition. Joe Louis famously fought the "Bum of the Month." Muhammad Ali, between his bouts with the likes of Joe Frazier and George Foreman, faced a collection of stiffs like George Chuvalo, a Canadian fighter with 17 losses to his name.

Every boxer has skeletons in his athletic closet. But when history is written it will show Mayweather not only beat his greatest contemporary rival, Pacquiao, but also the third-best fighter of his generation, Juan Manuel Marquez. While Marquez gave Pacquiao all he could handle, the Mexican great could barely lay a glove on Mayweather.

"I think that I don't get my credit that's due because I think that I make A-level and B-level fighters look ordinary," Mayweather told the media last year. "But that comes from just having a sharp mind and just really, really pushing myself in training—pushing myself very, very hard in training so when it's time to go out there and perform, everything is easy."

He may not have fought everyone—but Mayweather, according to Ring's Michael Rosenthal, fought enough greats to prove he's the real deal. If there was any doubt about his stature in the sport, the Pacquiao fight removed it for all time:

His many detractors might not like him or his safety-first fighting style, but they have to acknowledge his remarkable gifts and the grueling work he has put in to get the most out of them over two dominating decades. He has earned his reputation as the best pure boxer of his time and one of the best ever.

The case against Mayweather, of course, is much bigger than boxing. It's spilled over into the true mainstream, launching hundreds of think pieces.

Mayweather isn't your typical boxing bad boy, although he's that too thanks to years of reality television. He's also become a high-profile example of what's wrong with sports, joining football's Ray Rice as the athletic world's poster boy for domestic violence.

Putting filthy lucre into Mayweather's bulging pockets has become an increasingly bitter pill for many to swallow. Last September, espnW.com's Sarah Spain called for fans to boycott the fighter and send him the only message that matters—a monetary one:

How can one watch him box without picturing his fists pummeling not his opponent, but a defenseless woman? How can one acknowledge his fighting prowess without wondering how many women have fallen victim to it outside the ring? There might not be videos of Mayweather's attacks, but he has pleaded guilty on several occasions. Even though we haven't witnessed his abuse firsthand, is he not deserving of the same criticism and exile that Rice faces? Unlike Rice, Mayweather's character has been in question for as long as he's been famous. His public persona is egotistic, materialistic and bombastic, and his misogyny extends beyond physical abuse.

A beloved fighter might be able to get away with facing Andre Berto in his final fight. Fans would be happy to reward years of service in the ring with a glorified victory lap.

Floyd Mayweather is not that fighter.

It's impossible to have a completely impartial conversation about Mayweather. The battle lines have been dug deep, and emotion trumps objectivity every time. People, simply put, don't like Mayweather very much. And like it or not, that's going to bleed into how we remember him as an athlete—and how we react as he attempts to take his final bow.

Jonathan Snowden covers combat sports for Bleacher Report.