LAS VEGAS — When it as announced that Juan Manuel Marquez would defend his world lightweight title against Juan Diaz on July 31 the first question that popped into the minds of many fans and members of the media was: Why?

The two lightweights combined to make the 2009 fight of the year last February, a back-and-forth thriller that Marquez won by ninth-round stoppage. Since the first bout, however, both fighters have lost bouts. Marquez’s loss, a 12-round decision to Floyd Mayweather in a welterweight bout last September, was forgiven. It was obvious the 36-year-old fighter should not have fought anyone, let alone Mayweather, over 140 pounds.

Diaz, however, struggled to outpoint Paul Malignaggi in controversial fashion last August, and then soundly lost the rematch to the slick New Yorker via unanimous decision in December. Many believe that Diaz (35-3, 17 knockouts), who has lost three of his last five bouts, is not the same “Baby Bull” who once held three 135-pound titles.

So why should Marquez (50-5-1, 37 KOs), the lightweight champ who is still regarded as an elite fighter, give Diaz another chance?

The classy veteran answered that question at the press conference for the rematch, which will take place at the Mandalay Bay and be televised live on HBO Pay-Per-View.

“No. 1, he’s a great fighter and he deserves a rematch,” Marquez told the assembled media at the MGM Grand on Saturday. “No. 2, the people want to see this kind of fight. In 2009, our fight was the fight of the year. In 2010, maybe we will make the fight of the year again. We are two warriors in that ring, so it can happen.”

Richard Schaefer, CEO of Golden Boy Promotions, which promotes both fighters and put on the first match, certainly thinks so.

“Styles make fights and these two are made for each other,” said Schaefer, who also promised that his company would put on top-notch undercard to entice fans to come out to Las Vegas during one of the hottest months and to encourage hardcore fans to part with their money to watch the rematch live.

“This won’t just be fight of the year, it will be the night of the year, and you can hold me to this. We’re going to make great fights that could be their own main events on this undercard.”

Even with world-class supporting bouts, the July 31 main event might be a hard sell outside of Hispanic markets due to the perception that Diaz has seen better days and is one more loss away from being a stepping-stone for up-and-comers.

Diaz, who is only 26, is aware of the doubt the boxing world is directing at him, which is why he’s so motivated for this rematch.

“This is the fight I wanted more than any other,” he told RingTV.com. “Juan Manuel Marquez has been on my mind since our first fight. He was on my mind while I was training for both Malignaggi bouts.

“I was too emotional going into our first fight. I couldn’t even sleep days before the fight because I was so hyped to be fighting a legend in my hometown (Houston). It was like I was on Red Bull (energy drink) all the time. I came out to hard and it wound up biting me in the ass.

“That’s not going to happen this time. This time I’m going to listen more to my corner and use my jab better.”

Nacho Beristain, Marquez’s trainer, believes Diaz can box better than he did in the first bout, which is why he’s taking the young man seriously as an opponent.

“There was talk of Juan Manuel maybe fighting Ricky Hatton or Amir Khan at 140 pounds, but neither of those two are as dangerous as Diaz is, in my opinion,” Berstain told RingTV.com through translator Francisco Salazar. “Khan is not as tough as Diaz is. And who knows how much Hatton weighs? They said the fight would be at 140 pounds but he’s a light heavyweight right now.

“Diaz is still hungry. He’s a good opponent, which is why Juan Manuel will have to box better than he did in the first fight. I want to see more lateral movement from him in this fight.”

Better boxing? More jabs? Lateral movement?

Is it possible that the rematch could more of chess match than a fight of the year candidate?

“Not really,” said Diaz. “We might add some boxing to our strategy but we’re still going to fight. We have the kind of mentalities and hearts that always make for a hell of fight.”

Marquez agreed, explaining that their particular ring chemistry comes from their cultural heritage.

“He may live in the U.S. but he’s Mexican,” Marquez told RingTV.com. “I’m Mexican. As a Mexican fighter I will give my heart in the ring. So will Juan Diaz. I will spill my blood to keep my title and for the honor of my country.”