Anthony Joshua v Andy Ruiz Jr is make or break for Briton - Johnny Nelson

That is the verdict of Johnny Nelson, a former world champion in the cruiserweight division himself, but only after a few knock-downs along the way.

Now working on the big fights as a pundit for Sky Sports, Nelson believes the hitherto unbeaten Joshua was guilty of complacency and misjudged the seemingly out-of-shape Ruiz back on June 1 at Madison Square Garden, when the Mexican produced one of the biggest shocks in the storied history of the heavyweight division.

Ruiz knocked Joshua down four times en-route to a seventh-round stoppage that ripped the IBF, WBA, WBO and IBO belts from the 2012 Olympic heavyweight champion.

That surprise development temporarily halted any talk of potential mega fights with more vaunted heavyweight opponents like Tyson Fury and Deontay Wilder, and forced Joshua into tomorrow night’s rematch in the desert. But, provided Joshua exacts revenge, Nelson believes the whole experience will make the Briton a hungrier, more focused, individual.

“Anthony Joshua was probably getting to the stage where he was getting complacent and just going through the motions and getting the wins, and getting comfortable,” says Nelson, who retired as the WBO cruiserweight champion in 2005.

“If he was going to lose, the best time would be against Ruiz, rather than Tyson Fury or Deontay Wilder. So basically if he was going in with that attitude, he was better doing so against Ruiz.

“He’s got to right the wrong with Ruiz, but now he knows what losing as a professional feels like. So now when he gets in the ring with Fury or Wilder, if it does happen, he will not be at all complacent because he knows what defeat tastes like.

“It’s not about the money, now he’s got something to prove to himself.”

Many conspiracy theories have done the rounds as to what happened on that now-infamous night in New York back in June.

Illness, the change in training camp environment from Sheffield to Miami, or even a body double posing as Joshua, have been floated as explanations.

But for Nelson – a winner of 45 of 59 fights himself – the answer is simple.

“Joshua under-estimated what was in front of him. He went in there on one of those days and didn’t expect it. You can have bad days,” he says.

“You can’t get away with those mistakes in our game. Joshua went in there with the wrong attitude and he paid the price. It’s that simple.”

But even in making that assessment, Nelson offsets any criticism with an acknowledgment of just how quickly Joshua has risen. The Londoner didn’t take the sport up until 2007, when he was 18, and was an Olympic champion five years later.

His rise through the professional ranks was similarly meteoric, winning a world title in fewer fights than all but one boxer in heavyweight history – Leon Spinks – but quicker than Mike Tyson and Muhammad Ali.

“Look what he’s achieved in that short space of time, and people forget that,” offered Nelson.

“Anthony Joshua is strong-minded to have done what he has done in our sport, it’s amazing what he’s done in that short space of time.

“And people just don’t get it. Most kids box for 10, 11 years as an amateur, and then turn pro. Anthony Joshua has done it all in 11 years. It’s about mindset.”

That mindset will now be key in Saudi Arabia, as Joshua looks to gain revenge on Ruiz and put 2019 to bed.

So how does he do it? Does he stick to the tactics he employed in New York and hope to avoid Ruiz’s left hook? Does he try to slow-play Ruiz as Joshua Parker did when the big New Zealander inflicted the only defeat upon Ruiz two years ago? Or does he try something completely different?

“I think he’s got to choose completely different tactics,” says Nelson. “He’s got to box boring, it’s horses for courses.

“No one wants to see that but that’s what he’s got to do. It’s not about size, it’s about boxing ability. Look what Joshua did to Parker, because Parker outboxed Ruiz, so Joshua should be able to outbox Ruiz.

“There’s a lot of pressure on him, but there’s always been a lot of pressure on him. Anthony Joshua has boxed in front of 90,000 people. That’s pressure.

“But my goodness, this is make or break for him. Joshua knows the mistake he made, he knows what he needs to do to fix it.”