Fury, also a previous IBF, WBA and IBO champion, fights German underdog Tom Schwarz in a non-title bout in Las Vegas on Saturday night.
"It's been done many different times by many different fighters".
"Ask any top trainer who has been around the sport a lifetime".
Mexican-American Ruiz stopped 29-year-old Joshua in seven rounds at New York's Madison Square Garden in a major upset on June 1.
"If they [Team Joshua] want me to go over there [the UK] they've got to give me 50 million dollars".
Both men have stated where they would like it to take place, with Hearn claiming Joshua's side will have the final say. What were people seeing?More news: Journey through Greek mythology in Ubisoft's Gods & Monsters - out in February 2020
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He explained: "I'm usually quite jealous of how they all look, me being a fat man and they are all ripped, it's not something I watch to watch all the time!"
In response to these revelations, Ruiz Jr told Fight Hub: "Yeah, they did say November/December, but my team are negotiating everything right now".
But unbeaten Fury, who shed around 10 stone (64 kilograms) in a bid to re-establish himself in the heavyweight division after a battle with mental health and drugs issues, said body shape was no indicator of success.
Fury added: "Physically Joshua could out-train everyone".
"He would break every heart monitor, has probably worn out every treadmill in the gym, smashes the bag, it's all very unimportant". The result is a blow for the British heavyweight, who had plans to force a fight with either Tyson Fury or Deontay Wilder.
Ruiz Jr. wants the second fight against AJ to take place in the United States in Los Angeles or in Mexico.