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For a fighter in possession of two recognized world titles within a loaded 154-pound division with a chance at adding a third this weekend in a pay-per-view headlining role opposite Jermell Charlo, one would be hard pressed to find someone as comfortable being the underdog as Jeison Rosario. 

A native of the Dominican Republic, Rosario (20-1-1, 14 KOs) was a name even one year ago that most boxing fans had never heard of. Yet thanks to an upset stoppage of unified champion Julian "J-Rock" Williams in January, Rosario enters Saturday with a shot at becoming the power broker in possibly the sport's deepest and most competitive division. 

Not bad for a guy who can remember being the betting underdog -- a role he will play once more against Charlo (33-1, 17 KOs) -- for the last 10 fights running. The difference for Rosario is that he has been able to benefit from the perks of being overlooked and playing the spoiler without accepting the negative side of what the moniker implies.

"It's no pressure for me, it's more motivation," Rosario told CBS Sports last week through a translator. "I have always come in as the underdog even though I don't think as one. If I am supposed to lose, no one told me the script. They didn't tell me that. Somebody forgot to tell me the script that I'm supposed to lose in this fight. I'm used to it and there is absolutely zero pressure on me. This is just another night where I will be able to showcase my talent."

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By simply relying on boxing math and common opponents, there's no wonder why Rosario would be deemed an underdog against the 30-year-old Charlo, whose combination of power and speed has made him the division's best fighter of the moment. Regaining his WBC title via stoppage against Tony Harrison in their December rematch certainly helped that notion. 

So did the fact that Rosario's biggest win before the Williams upset came via split decision against a fighter in Jorge Cota whom Charlo destroyed via explosive knockout in 2019. Rosario also owns a 2017 loss to Nathaniel Gallimore, a tough fighter who has lost four of his last five against the likes of Williams, Erickson Lubin, Sebastian Fundora and current WBO champion Patrick Teixeira. 

Yet if you're looking for a reason, outside of his powerful right hand, to brand Rosario as a live dog against Charlo, think about this for a second: Not only is Rosario just 25 and seemingly getting leaps and bounds better with each fight, a reason for that just might be that the Williams fight marked the first time Rosario had ever done a real training camp as a professional. 

Rosario was so overwhelmed with emotion after absorbing big blows and stopping Wiliams on his feet in Round 5 that he mentioned during the post-fight interview how he would've committed to proper training so much earlier in his career if only he had known these type of results were possible.

It's that kind of gleeful ignorance, fueled by a want to provide for his wife and children the things he could never dream of as a poor youth in Santo Domingo, that has made Rosario's Cinderella story so much fun to watch. It has also gone a long way in making the fight so much harder to confidently handicap considering Rosario — let alone boxing as a whole — simply has no idea how good he can become.  

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"I never thought at 25 I would be here," Rosario said. "I learned from my mistakes and got the opportunity to fight J-Rock. I did a training camp for the first time because I felt that was what we needed to do. You guys saw the result. I never imagined it and am grateful for this opportunity. I'm going to take full advantage of everything opened up to me."

Should Rosario pull the upset on Charlo, one would have to believe it would come as a result of letting his hands go freely. Growing up in his baseball-crazed home nation where sluggers of both baseball and boxing often credit their power to a heavy diet of a certain superfood, Rosario has kept the "Banana" moniker and wears the nickname like a badge on his sleeve to warn opponents of his power. 

"If I land the big banana, I don't think [Charlo] or anybody can tend to that," Rosario said. "I'm coming with two bombs in my hand on this fight and I'm going to put it all on the line for the big prize. I am sure he will be doing the same and will come in prepared like never before. But if I land the big banana, not he or anybody will be able to take it."

Although Rosario said he will box Charlo should the situation demand it, there's no question he is prepared for war. It's not only what he claims to know how to do best, it's a style he prefers. 

Rosario said his confidence was also buoyed by watching Charlo compete so closely in a pair of title bouts he split against Harrison.

"I know Charlo is regarded as the No. 1 154-pound fighter in the world. He earned that and I respect it but what I learned from the [Harrison] fights is that Charlo is not invincible," Rosario said. "Maybe the promoter has been trying to make him look good but he's not invincible. He is beatable and a fighter who can lose, just like anybody else."