LAS VEGAS — Amir Khan, an English-born fighter of Pakistani descent, said on Thursday that Ramadan would prohibit him from fighting Floyd Mayweather Jr. on Mayweather's traditional September fight date should both fighters get beyond their bouts on Saturday night at the MGM Grand on Showtime Pay Per View.

Khan will face southpaw Luis Callazo in a clash of former titleholders on the undercard of Mayweather's RING/WBC welterweight title defense against WBA counterpart Marcos Maidana, who was chosen as Mayweather's May 3 opponent over Khan.

Maidana represents the third fight of Mayweather's six-bout deal with Showtime, who traditionally competes in May and September — the two Mexican holidays, Cinco de Mayo and Mexican Independence Day.

Ramadan dates generally shift from year-to-year based on the Islamic calendar. During Ramadan, Muslims fast from dawn to sunset, including a prohibition on drinking liquids.

"Ramadan is going to be right in the middle of the year, so I think that can affect a fight with Floyd, if we both get through on Saturday. I can't take that fight. I need a couple of months to work on my strength and conditioning," said Khan, during an interview with MLive.com.

"If Floyd wants to fight in November sometime, if it can't be done in September, then I'm happy for November. If not, then it'll have to happen in the future. If it never happens, everyone will have to just think, 'What would have happened if I had fought Floyd?'"

It will have been more than a year since Khan has fought when the 27-year-old enters the ring against Collazo, who is 6-1 with four knockouts in his past seven fights, and who is coming off January's second-round stoppage of former titleholder Victor Ortiz.

While Khan (28-3, 19 knockouts) owns a unanimous-decision victory over Maidana (who scored knockdowns in the fourth and eighth rounds of a unanimous decision that dethroned Adrien Broner as WBA welterweight titleholder in December), he has relatively struggled over the course of his past six fights, going 4-2 with two knockouts.

In the 33-year-old Collazo (35-5, 18 KOs), Khan will make his 147-pound debut against a career welterweight who has gone 6-1 with four knockout victories in his past seven fights.

Collazo is 4-0 with two stoppage wins since falling by 10-round unanimous decision to Freddy Hernandez, who dropped him in the eighth round in October 2011.

Khan said he has been physically draining himself to make 140 pounds since way back in July 2011, when he stopped ex-beltholder Zab Judah in the fifth round of their IBF/WBA junior welterweight title unification bout.

After defeating Judah, Khan lost his next fight by disputed split decision to current IBF 140-pound beltholder Lamont Peterson in December 2011 and was stopped in the fourth round in the following bout by RING, WBC and WBA junior welterweight champion Danny Garcia in July 2012.

Khan left trainer Freddie Roach in favor of Hunter for his next two fights, resulting in a 10th-round knockout over Carlos Molina, a California-based lightweight prospect, in December of 2012. That was followed by a unanimous decision over former titleholder Julio Diaz, who dropped him in the fourth round in his last fight on April 27. Molina will fight Broner on the Mayweather-Maidana card.

Khan's resume boasts a technical decision victory over Mexican great Marcos Antonio Barrera as well as a TKO over two-division titlewinner Paulie Malignaggi and a decision over ex-beltholder Andriy Kotelnik. Khan earned major 140-pound belts by beating Kotelnik and Judah.

In addition to his loss to Garcia and the fact that he was dropped and badly hurt by Julio Diaz, Khan has endured lingering questions about his chin as a result of a 54-second KO loss to Breidis Prescott in September 2008.

"I've been in with the best, and every time I get knocked down, I get back up again," said Khan. "I've been put down, but I got back up again. I've then gone on to continue winning fights and I've been stopped, but I was on my feet."

Khan referenced eight-division titlewinner Manny Pacquiao, who rebounded from a sixth-round knockout loss to Juan Manuel Marquez that left him lying face-first and unconcious in December 2012 with consecutive unanimous decision victories over Brandon Rios and Tim Bradley in November and last month, respectively.

"I've never been knocked out, like, knocked out. Like Manny Pacquiao got knocked out cold, but does that mean that he has no chin? There have been many fighters who have been knocked down," said Khan.

"But because many people want something against me, they want to put something on me to say that I've got a weak chin. I've been in with some big punchers, and the guys that have put me down have been big, heavy-handed guys."