B.J. Penn is a black belt in Brazilian jiu-jitsu. Nick Diaz is a black belt in Brazilian jiu-jitsu.

Nick has fought professionally as a boxer and trained with world champion and Olympic gold medalist Andre Ward. B.J.'s standup skills have caught the eye of Manny Pacquiao's cream-of-the-crop trainer, Freddie Roach, who called Penn "by far the best striker in MMA."

Yes, the main event of Saturday night's UFC 137 in Las Vegas (9 p.m. ET, PPV) is a rare matchup in which both competitors are equally adapt and comfortable whether a fight is standing or on the ground. It makes for an intriguing rumble.

Experts' predictions for UFC 137

Does that make up for the fact that this is a third-fiddle top attraction? After Diaz initially was yanked from a welterweight title bout with Georges St-Pierre on this card because he failed to show up for a pair of promotional appearances, and his replacement, Carlos Condit, was deprived of a shot at the belt when GSP injured a knee in training, we were left with an event that's lost some luster.

For fans being asked to plunk down $54.99 for an evening of fighting, the caveat emptor concern is not merely with the absence of St-Pierre, one of the sport's most accomplished and popular athletes. On top of that, the UFC has added no value back into the deal. Penn vs. Diaz is a nice backup main event, but it's a bout that already was on the card. So with GSP vs. Condit disappearing into thin air, without replacement, are paying customers getting bang for their buck?

UFC president Dana White thinks so. (Of course, he's paid to think so.) He has pointed to the typically deep UFC 137 card as an example of how his fight company's matchmaking surpasses boxing's. And yes, it's true that a cancelled main event surely would collapse any sweet science fight card. But Dana might want to reconsider whether he ought to be stacking up his business model alongside -- or in any way associating it with -- the nonsense that goes on under the backward-thinking watch of promoters like Bob Arum.

And, really, despite all of Dana's hype about how his undercards pack a punch (and despite all of my bellyaching above), the UFC is pretty top-heavy, too. It's nowhere near as extreme as in boxing, but a superior main event is the biggest selling point for an MMA PPV. So what is going to sell you on UFC 137 -- or not sell you -- is Penn vs. Diaz.

What do we have on Saturday night, then? There is no belt on the line and there are no immediate title implications, although an impressive winner could very well jump to the head of the line at welterweight, leaving the patient Condit to helplessly and fruitlessly clutch his No. 1 deli ticket. More than anything else, though, there are two things that make Penn vs. Diaz a bout worth your time and money: Penn and Diaz.

These are two jiu-jitsu virtuosos whose ground games couldn't be more different. Penn grapples to gain a dominant position or escape a bad one; Diaz plays some of his best offense while calmly on his back. As a striker, Nick wears you down with an incessant pitter-patter that leaves no room for response, while B.J. can play that game or he can explode on you. Diaz lands twice as many strikes as Penn, according to FightMetric stats, but for the past five years he's been landing them against Strikeforce talent, not UFC. That last factor makes Penn's wrestling edge more eye-opening: He's been successful on nearly 60 percent of his takedown attempts, while Nick nails only a third of his. That means B.J. should be able to dictate where this fight is fought.

Diaz's biggest advantage is in conditioning. The guy trains like a triathlete moonlighting as a fighter. However, in a three-round bout, how much will cardio come into play?

In the end, the determining factor in this fight might be not so much physical as mental. Who's more motivated? When you're considering factors involving the mind, of course, you've got to look closely at the fragile, enigmatic Diaz. He was to be competing for a belt, but now he's not. He's fighting a friend, someone he once idolized and emulated. And this fighter who so often talks about money is not making as much of it as he would have in a GSP bout. Sure, Nick could enhance his earning power with a great performance this weekend, but to strategize that way would involve forward thinking, and Diaz seems to be more the type of guy who lives in the moment -- not necessarily the present moment, but surely not the future, or at least a fighting future.

"It could maybe come down to who wants it more," Diaz acknowledged in an interview with the HD Net show Inside MMA. "And at this point in time, that could be my biggest issue, you know? 'Cause I've been doing this for a while, and I don't need this. But I'm a fighter, and I'm here to fight. I don't back down from fights."

2: Weight classes in which he's won UFC titles (lightweight, welterweight).

59: Percent of successful takedowns over his career.

0: Number times he's been submitted in his 25 fights. (Diaz hasn't been subbed, either, in 33 bouts.)

84: Percent of his 25 wins that have come by knockout (13) or submission (8). (Thirteen of Penn's 16 wins have been by KO or sub.)

10: Consecutive victories.

6: UFC wins (against four losses) before moving on from the promotion in 2006.

What we should expect: With these guys' versatile skills, there's the potential for some magical moments -- slick scrambles on the mat, crowd-rousing fisticuffs while Nick and B.J. are standing and trading. Both fighters have the tools to finish, and both have the know-how to avoid being finished. The X factor, to my thinking, could be the poise and maturity of Penn, who'll press to take the fight where he needs it to be for him to have the best chance of winning. Diaz, on the other hand, is prone to challenging himself by eschewing safe positions in favor of trying to beat an opponent at his own game. That's risky business against a fighter as crafty, proficient and experienced as B.J.

Why we should care: There's no championship on the line (although Diaz has said he never bothers with his Strikeforce belt anymore and Penn is welcome to have it), and it's the fight card's third stab at a main event. But how often do we get to see two fighters capable of competing at the highest level no matter what positions they find themselves in?

" ... ." --Nick Diaz during the first 45 minutes of a recent UFC 137 conference call with MMA media (he was a no-show until calling in late)

"Nick is Nick. That's what he does. I enjoy watching the stuff that Nick Diaz does. He doesn't change. He's always himself. He always shows up to the fight." --B.J. Penn, during the same conference call, responding to a question about Diaz' absence

"No, I'm not happy about it at all. I'm fighting this guy who was my friend. I was supposed to fighting this other guy." --Diaz, after joining the call, referring to "this other guy" named Georges St-Pierre, his original opponent until he no-showed two press conferences

"You know what? I think Carlos Condit and Georges St-Pierre is a great fight. But I think this fight is the real main event." --Penn, in a UFC-produced hype video shot before GSP's injury pushed Penn-Diaz to the top of the bill

"I think B.J.'s a lot better fighter than GSP, to be honest. He's a lot better at jiu-jitsu and boxing, all the way around." --Diaz during the conference call

"I really feel that Nick is the best boxer in mixed martial arts today. He has the highest punch volume. He can stand up with anyone." --Penn during the conference call

"For this fight, my biggest obstacle is overcoming my own personal faith in B.J. Penn. I don't look up to a lot of guys, but I've looked up to B.J. Penn for a long time." --Diaz in an interview on the HD Net show Inside MMA

Where's the Meat?: Matt Mitrione is unbeaten and on the rise, his fight with Cheick Kongo earning the co-main even slot when Penn vs. Diaz was moved up. To grasp the challenge at hand, all Mitrione needs to do is watch Kongo's last fight. The big striker was hurt early by a punch and nearly finished by Pat Barry, which would seem to bode well for Mitrione. But "Meathead" should keep watching and take heed, as a reeling Kongo put an emphatic end to the bout when he connected with a desperation uppercut and laid out Barry.

Too many unhappy returns: I'm eager to see Roy Nelson back in the octagon. Mirko Cro Cop, at age 37, not so much. He's lost his last two fights, both by knockout, and you can already see the next one coming, even though "Big Country" hasn't yet thrown the punch.

Welcome to the big leagues: Hatsu Hioki has won 24 of his 30 career bouts, losing just once in his last 14. He's been a champion in the Shooto and Sengoku fight promotions in his native Japan, and is ranked highly among featherweights by many MMA publications (including No. 3 in SI.com's latest ratings). But when he steps in against George Roop, he'll be an octagon rookie.

SI Apps
We've Got Apps Too
Get expert analysis, unrivaled access, and the award-winning storytelling only SI can provide - from Peter King, Tom Verducci, Lee Jenkins, Seth Davis, and more - delivered straight to you, along with up-to-the-minute news and live scores.