In 1996, Zab Judah was supposed to represent the United States in the light welterweight division at the Olympic Games in Atlanta. Somebody got in his way. Nobody could quite believe it.

Ten years later, Jose Armando Santa Cruz was supposed to win the WBC interim lightweight title at the Thomas and Mack Center in Las Vegas. Somebody got in his way. Nobody could quite believe it.

A year after that fight, Mexican legend Erik Morales was supposed to start his final comeback by winning the WBC lightweight title at the Allstate Arena in Rosemont, Ill., just outside of Chicago. Somebody not only got in his way, he sent Morales into retirement. Nobody could quite believe it.

This pesky person nobody seems to believe in is now preparing to face Manny Pacquiao for that same version of the 135-pound title in Pacquiao’s first foray into the lightweight division. Manny Pacquiao is supposed to win when he steps into the ring at the Mandalay Bay Events Center on June 28. If he doesn’t, nobody will quite believe it.

Nobody but David Diaz, who has made a reputation and finally a few dollars by upsetting other people’s apple carts as well as other people’s fighters. Diaz is a humble guy by nature and so would be the first to tell you maybe he’s nothing special in the world of prize fighting. He’ll tell you that but it doesn’t mean he won’t intend to beat your ears back after he’s through talking.

That’s because he’s learned what so many in boxing never have understood. He’s learned that all the talking in the world and all the expectations of the world don’t mean a thing once the fighting starts. And fighting is something he’s quite willing to get involved with, even against someone universally recognized as one of the best pound-for-pound fighters on this planet.

“I’ve been wanting a big fight like this for a long time,’’ Diaz (34-1-1, 17 KO) said from his home in Chicago last week. “I feel pretty good about this fight. Pacquiao is a top fighter in his prime but when I look at him I see a guy I can hit real easy. I see a guy I can beat. I honestly feel that.’’

That would put Diaz in a very small minority in boxing,  for Pacquiao has long been viewed as one of the very best fighters in the world after winning world championships in three weight classes (flyweight, super bantamweight and super featherweight) and beating up so many Mexican fighters he has been labeled the Mexican Assassin by less than pleased Mexican boxing writers.

Meanwhile, Diaz has long been looked upon as the B side in all of his biggest victories dating back to when Judah was the heavy favorite to win a slot on the Olympic team despite Diaz’s 175-16 amateur record. That upset didn’t seem to change many opinions of Diaz and the ones that have followed haven’t either.

By now this might be a bit insulting to some fighters but Diaz long ago outgrew such an outlook. To him, there are only a few people picking against him that would affect how he sees himself or how he sees his chances against even someone like the popular and heavily-favored Filipino.

“It’s the way it’s been for a long time,’’ Diaz said of his constant role as a fistic underdog. “By now you’d think some people would give me a little bit of a chance but that’s all right. The only thing that really matters is when your hand is raised.

“I’m pretty good at blocking out that sort of negative stuff and I stick to that. I’ll do everything possible to be ready. After I beat him maybe we can put a stop to that kind of thinking but I don’t get insulted by it. I really don’t. I don’t care about those people (who say he has no chance against fill in the blank). I don’t talk to them all the time.

“If my wife or my brother said ‘Pacquiao’s going to beat you’ then we got a problem.’’

They are saying no such thing of course. Instead they are holding fast to a faith in Diaz that took a decade to be rewarded but finally was when he came from behind to stop Santa Cruz and win what was then the WBC interim lightweight title. Later he would become the fully recognized champion after Joel Casamayor failed to make weight in a fight with Diego Corrales 14 months ago, and he would get his first big win by outfoxing and outfighting Morales last August to retain a title he was never supposed to have won in the first place.

Now he stands to take another step up boxing’s ladder, one leading to his biggest payday,  but one that is supposed to be little more than a well-paid fistic funeral for him. Even Diaz’s promoter, Bob Arum, has said as much.

When Pacquiao barely escaped with a split decision in his rematch with Juan Manuel Marquez there was pressure from a lot of corners for an immediate rubber match. Arum refused, saying there needed to be some “air’’ between Pacquiao and Marquez before they squared off a third time with millions of dollars at stake. The implication was Pacquiao could get such air facing Diaz in a low risk confrontation that would allow him to adjust to fighting at 135 pounds before taking on the more formidable Marquez a third and final time.

This might not have sounded so bad had Arum not been Diaz’s promoter. More than a few boxers would have been mighty peeved at Arum for representing them in such a fashion but Diaz understands Pacquiao fights for Arum too and has produced far more income for him. In the bottom line business of boxing, that’s what makes alliances.

“I understand,’’ Diaz said. “It’s business. Manny is the guy bringing in all the chatter. If you let that offend you it disturbs everything you’re trying to do. You start worrying about the wrong things.

“He gets the filet mignon and I get the New York cut but that’s better than nothing. The important thing is winning the fight. When I beat him I’ll be the guy they’re talking about. Until then, if you take that kind of thing personally you corrupt your heart.’’

That is one thing Diaz will never do because his heart is in large measure his greatest fighting asset. He is not a terrific puncher or the greatest boxer in the division. He is, some will argue, just a journeyman. He would call himself something else.

David Diaz would call himself a professional, which means something in boxing. It means he comes to the arena ready to win and with the intention of bringing bad intentions with him into the ring. He comes with a belief in himself he knows few outside his corner share but which he understands is all that really matters.

Zab Judah had more believers than he did. Santa Cruz had more believers than he did. Morales had more believers than he did. So there is no surprise to learn the same is true with Pacquiao, who most observers think is second only to Floyd Mayweather, Jr. on the mythical pound-for-pound list.

Frankly, Diaz might put him there himself. But that doesn’t mean he won’t arrive in Las Vegas in two months intending to put him on his back.

“What Pacquiao does is make you fight his type of fight,’’ Diaz said. “In my case that’s fine because his type of fight is my type of fight. Pressure, that’s my best offense and my best defense.

“He’s an all-out type of fighter. He throws in volume so I have to match that. Punching power wise I don’t know who’s stronger but my determination will be the difference. This fight means more to me than it does to him.’’

What Diaz believes is that Pacquiao will come to Las Vegas with the same attitude Morales had when he came to Chicago. With fighters like that, fighters who have for so long been among boxing’s elite, there is a natural cockiness, a dismissive way they look at guys who have come up the way Diaz has, which is the hard way.

That does not mean they don’t train for them or that they don’t respect them on some level but there is a clear belief that such an opponent is not really on their level. Many times they are right. Sometimes, as Morales found out, they are not but that knowledge often comes too late.

What Manny Pacquiao thinks of him David Diaz does not know. Nor does he care. All he cares about is that he has two more months to get ready to show him he’s more than the world thinks he is.

“Look, this is a tough fight,’’ Diaz said. “I know Pacquiao can fight. But I don’t know any fighter who doesn’t want to be in this type of fight.

“This is the kind of fight people will talk about long after you’re done and gone. This is the biggest fight of my life. Bigger than when I won the title. Even bigger than beating a legend like Morales because Pacquiao is a guy in his prime. He’s a guy who’s been beating up people in different weight classes. Now I have a title he wants and he’s trying to put me on the mat. It’s one of those fights where you got to prove yourself to yourself.

“That’s really all I care about. Proving to myself what I can do in boxing. Becoming champion at one time was something I didn’t think I’d achieve. I thought maybe I was just another pro. Now I know I can be more than that. I’m not content with what I have.

“I’m finally getting paid what champions get paid. This is a fight where I can set up my kids for college. It’s about making their life better but win this one and next fight maybe I can set up David Diaz.’’

That’s why he’s up every day before dawn running in Chicago’s early morning light. It’s why he leaves the house by mid-morning for the Jab Gym, where he trains for two to three hours. It’s why when he comes home he spends more time thinking about Manny Pacquiao than he does about himself.

Halfway across the country, in a gym in Hollywood, CA. where Manny Pacquiao has begun training, Diaz’s opponent is thinking about many things. Not very often, Diaz believes, is he thinking about him.

That doesn’t insult him either. He’s used to that too. Just like he’s used to leaving the ring after the battle has been fought with a lot of skeptics saying, ‘How’d that happen?’’

“If he saw my fight with Morales he’ll be in shape,’’ Diaz said defiantly. “If not, that’s his fault, not mine. I’m coming to win.’’