When Floyd Mayweather Jr. abruptly retired three weeks ago, he left a vacancy atop an imaginary list.

From sports bars around the globe to the Internet's innumerable boxing crevices, fight fans love to argue about their hypothetical rankings of the world's best pound-for-pound boxers. With the Pretty Boy out of the picture, many believe Manny Pacquiao is the man to assume that top spot.

After all, the Philippines' national icon has the title belts and the career longevity to deserve that invisible crown, and the sometime movie action hero creates an excitement that can't be matched with his frenetic, violent grace.

Pacquiao will go after David Diaz's WBC 135-pound title at Mandalay Bay on Saturday night, hoping to set an impressive career milestone with a major championship in his fourth weight class. Few fighters have ever succeeded in such physical growth, and Pacquiao is all the more remarkable after starting out at 107 pounds.

Yet after everything he has accomplished in a career that's risen faster and higher than anybody expected, Pacquiao won't promote himself for that pound-for-pound crown. Diaz gives him more than enough to worry about this week.

"I would just say thank you to people for believing in me, pound-for-pound," Pacquiao said. "All I'm trying to do is make people satisfied with my performance, not only in the ring, but as a champion outside the ring. That's what I want to do the most."

Top Rank's Bob Arum, who rarely hesitates to promote anything that might draw attention to his fighters, also has no interest in crowning Pacquiao as the ultimate champion _ even after The Ring magazine selected Pacquiao its top pound-for-pound fighter this month, as did Yahoo Sports in its poll of media members.

"Obviously, Manny is a tremendous fighter, a very exciting fighter," Arum said. "But I don't know what it means to be the best pound-for-pound fighter. I don't think any of these other jokers do, either. I think they make it up. You take Pacquiao, and you take (Kelly) Pavlik, and you compare them. What does it mean? It means nothing."

Arum promotes three of the four fighters most frequently mentioned in the post-Mayweather pound-for-pound discussions: Pacquiao and unbeaten titleholders Miguel Cotto and Pavlik, who both can't match the length of the 29-year-old Pacquiao's resume. Joe Calzaghe, the unbeaten Welshman who reigns as the world's unofficial best at both 168 and 175 pounds, is the other favorite for the top spot, but he plans to fight just once more against Roy Jones Jr. before retiring.

"It's very hard for me to determine what goes into being the best pound-for-pound fighter," Arum said. "There are different weight classes, different categories of fighters. I don't know how you compare a Pacquiao to a Cotto to a Pavlik. What's the basis for comparison?"

Most indulgers in these hypothetical ramblings suggest the fighters' overall record, title belts, entertainment value and general reputation, to name a few criteria. Arum believes such a formula is far too complicated and subjective to come up with a concrete answer.

Still, Pacquiao's next fight could provide another remarkable line on his loaded list of credentials. Diaz is a former U.S. Olympian and a well-liked champion, but he's lightly regarded by many fight-watchers who don't think his skills are on the same level as top lightweights Nate Campbell, Juan Diaz and Joel Casamayor.

If Pacquiao wins, he'll be a world champion at his fourth weight class, adding to his belts at 112, 122 and 130 pounds. He took Juan Manuel Marquez's super featherweight belt in his last fight in March, winning a razor-thin split decision in what's sure to be one of the year's most exciting bouts. Pacquiao has fought nearly all of the top challengers in every weight class, and he still hasn't lost his trademark speed while growing in size.

If Pacquiao loses to Diaz, or if he even struggles against the lightweight champion, the Filipino star will slip down those mythical pound-for-pound rankings. Pacquiao simply doesn't care _ and he's thinking much more about the victims of the typhoon that capsized a ferry in his native land and killed hundreds last Saturday.

"I'm dedicating the fight to them," Pacquiao said. "I feel sad because of what happened, but I have to focus on training. If I have a chance, I'm going to help when I get back."