Steve Bunce explains to ESPN what Anthony Joshua needs to do in order to secure a unification fight with Deontay Wilder. (2:37)

NEW YORK -- Anthony Joshua, the British megastar, has come to the Big Apple looking to defend his three major heavyweight world title belts and make a splash in his U.S. debut.

It will be a bit different than he initially planned when he steps into the ring to make his seventh defense on Saturday (DAZN, 9 p.m.) at Madison Square Garden. Joshua will fight contender Andy Ruiz Jr. instead of undefeated New Yorker Jarrell "Big Baby" Miller, who was bounced from the fight about a month ago after failing four Voluntary Anti-Doping Association-administered random drug tests for three different banned substances -- GW1516, human growth hormone and EPO -- and being denied a boxing license by the New York State Athletic Commission.

Matchroom Boxing promoter Eddie Hearn and Joshua's trainer/adviser Robert McCracken started looking for potential replacement candidates. After contender Luis "King Kong" Ortiz turned down the fight, Ruiz, who had asked for a fight with Joshua after knocking out Alexander Dimitrenko in the fifth round on April 20 in a great performance, was selected as the new opponent.

Ruiz put his signature on the dotted line and Joshua will face an opponent who is not much of a downgrade from Miller, if he is at all.

Joshua, who is coming off a nine-month layoff since knocking out Alexander Povetkin in September, did not hesitate when Ruiz was brought to him as the new opponent.

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"I think Andy is a great challenger and will bring it, and the show goes on," Joshua said. "My initial response [after Miller was booted from the fight] was, 'Who's next?' Eddie isn't going to call me and say 'Miller is out' for a joke.' He's busy and he's not going to waste my time either. He's a straight-up guy. He called me and said, 'Are you alone? Miller has failed a drug test.' I said, 'OK, what's the situation? We've got to find a new opponent.'"

Ruiz happily took the fight and is aiming for the upset.

"There's a lot of pressure on me, but I think Anthony Joshua has more to lose fighting a dangerous fighter like me," Ruiz said. "I am a better opponent than all of the other opponents they were talking about. I am here to win it all.

"Everything happens for a reason, and what happened to Jarrell, I'm here to shock the world, take the belts and make history."

This is your Ringside Seat for the fight:

Second chance

Andy Ruiz Jr., right, could become the first Mexican heavyweight world titleholder with a victory over Anthony Joshua on Saturday. Ed Mulholland/Matchroom Boxing USA

Ruiz (32-1, 21 KOs), 29, of Imperial, California, is bidding to become the first Mexican heavyweight world titleholder. He has had one shot before but lost an extremely close majority decision to Joseph Parker when they met for a vacant title on Parker's home turf in New Zealand in December 2016.

Ruiz, who has since won three fights in a row, is a massive underdog, but he's fearless and poised.

"What I love about Ruiz is he doesn't realize what's about to happen," Hearn said. "He don't care. He's like, 'Hey, man, I'm going to Madison Square Garden and I'm gonna knock this f----- out.' He's not thinking there are gonna be 10,000 Brits there. He's going to have zero fear. He's not going to go in there and crumble. He's going to go in there and have fun, and that's dangerous. It's not a good style for AJ -- a shorter guy with good movement and fast hands."

Ruiz, who is 6-foot-2 and 250-plus pounds, said he has been underestimated because of his physique. But he knows exactly what is at stake and how fortunate he was to luck into the fight.

"This is a lifetime opportunity," he said. "This is my second chance to make history, to make my dreams come true. It couldn't have happened any better than now.

"I just fought April 20. I'm sharp. I'm prepared. We don't have to start a new training camp. It just keeps going. I'm just really excited. Everybody is underestimating me, just the way that I look, but I have heart, I throw a lot of punches. I'm gonna show the world what I am and what I could do. I'm gonna prove everybody wrong. I'm gonna bring those belts back to Mexico. I'm gonna make history."

Joshua (22-0, 21 KOs), 29, who is 6-6 and a chiseled 255 pounds, knows how important a memorable performance is for him in his American debut, so he swears he is not looking past Ruiz with so many bigger potential fights on the horizon.

"All I hear is 'AJ will smash him in a round,' but I don't listen," Joshua said. "From the outside they think that anyone can box, so how is the guy that doesn't look like a fighter able to box? I always say that you put 10 bodybuilders in the ring, not one of them could fight for a regional title. It's not about what you look like. It's a craft, a skill, and what's in your heart and your head.

"Andy has all that. He can fight and box, that's what matters. Take me out of my body but keep the same attributes, same jab, same chin, same heart and same mind, but a different body ... I'd still get to the same position I am in because it's what is within you that makes a champion, your genetics, and his genetics are the same -- and he took the fight!

"He's keen, he's game and you cannot knock him. He can fight and he's got hands. He gave a world champion in Parker problems. Andy is championship level for sure. I have not underestimated him one bit. Being famous and popular doesn't win fights. Knowing how to box and fight wins fights."

McCracken said Joshua took training as seriously after Miller dropped out as he had been before.

"This is a test for AJ," McCracken said. "Ruiz is [ranked] by all the people in boxing. I [rank] him myself. I know he can fight. Appearances mean nothing. AJ knows Ruiz is a good fighter and he knows he's a real test. So he's trained diligently and shown Ruiz respect in his preparation."

No Joshua-Wilder

Heavyweight world titleholder Deontay Wilder, left, seems to be headed to a rematch with Luis Ortiz later this year. TIMOTHY A. CLARY/AFP/Getty Images

The elephant in the room is the fight that is not happening -- the undisputed title showdown between Joshua and American Deontay Wilder (41-0-1, 40 KOs), who have been calling each other out for more than a year.

There have been efforts to make the fight, and at various times each side has looked like the one standing in the way.

There was faint hope that Wilder's first-round knockout of mandatory challenger Dominic Breazeale on May 18 combined with a Joshua win on Saturday would pave the way for the big one this fall.

This week, Joshua said he wanted the fight next and that he hoped to take a page out of the Floyd Mayweather-Manny Pacquiao book by bypassing handlers and meeting personally with Wilder to see if they could come to an agreement for the biggest fight in boxing.

Joshua turned up on ESPN's First Take on Tuesday and explained, "It will be good for me and Wilder to sit down and talk and get this fight negotiated. ... I've just come to the decision it's important for me and Wilder to sit down man-to-man and iron out our differences and get this fight made."

play 1:25 Joshua has 'no fear' in defending titles vs. Fury, Wilder Anthony Joshua joins First Take to issue a challenge to Tyson Fury and Deontay Wilder, whenever they are ready.

Nice idea, but alas, that fight is out the window for this year no matter what happens Saturday. This time Wilder takes the blame for the fight not happening, because hours after Joshua's appearance on First Take, Wilder announced that he had signed for a fall rematch of his 2018 fight of the year contender against Ortiz. Wilder didn't even bother to wait to see what happened with Joshua-Ruiz and make another effort to make the fight.

So should Joshua defeat Ruiz, he's probably looking at making a mandatory defense against Kubrat Pulev next instead of fighting for the undisputed championship.

By the numbers

Tale of the Tape Joshua Category Ruiz England Country USA 29 Age 29 22-0, 21 KOs Record 32-1, 21 KOs 6′6″ Height 6'2" 82" Reach 74" Orthodox Stance Orthodox

Joshua: Third English fighter in history to hold three major belts simultaneously (the only one to defend them)

Joshua: Making the fourth defense of the unified heavyweight title (second since he won his third belt in March 2018)

Joshua: 95.5% KO percentage is second highest among current titleholders and highest all time among heavyweight titlists

Joshua: Landed 31% of his jabs in his last eight fights (one of three fighters to land 30% or more -- Golovkin, 31.6%; Wilder, 30.1%)

ESPN Stats and Information

Ruiz: Lost his only previous world title fight in 2016 against Joseph Parker (who was then beaten by Joshua after two defenses)

Ruiz: Last fought on April 20; 42-day turnaround before challenging for heavyweight belt is fifth shortest in the last 50 years

Ruiz: Looking to become first heavyweight champion of Mexican descent (previous seven have lost)

Ruiz: In his last three fights landed half of his total punches to the body (6.6 per round)

Betting odds (Caesars Sportsbook): Joshua -2000 (favorite), Ruiz +1000 (underdog)

Playing the Garden

Anthony Joshua, right, has trained very hard for this fight, according to trainer Robert McCracken, left. Ed Mulholland/Matchroom Boxing UK

Many of boxing's biggest stars have fought at Madison Square Garden, the famed "mecca of boxing." Joshua will be another in a long line of U.K. fighters to do so, including Hall of Famers Lennox Lewis, Joe Calzaghe and Prince Naseem Hamed.

If Joshua, who regularly draws sellout crowds of around 80,000 at stadiums in the U.K., was going to come to the U.S., the Garden was where he wanted to box. But he is trying not to get too caught up in the hoopla of the event.

"I am just rolling with the punches and not trying to get caught up in the hype of it all. The main time when you enjoy it is after the fight," Joshua said. "God willing, I win. Then I will look and say, 'Wow, I've really done it at Madison Square Garden.' I just don't want to take part [in it], I want to own the night. And that's why I need to win."

McCracken endorsed Joshua's trip to fight in New York.

"This was the time to come to the States, fight a live opponent in Ruiz, a real contender, and show what you can do to the American audience," he said. "Madison Square Garden, historically, is huge in boxing. Every fighter who understands the history of boxing wants to fight there. It doesn't get any bigger than Madison Square Garden. Historically, that's the place to fight. He understands that and boxing history."

Rafael's prediction: Joshua by late knockout.