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.
The big Briton said Joshua had shown against Ruiz that he did not want to be in the ring.
“When a man doesn’t want to be there once, he will always do it and it’s hard to come back from,” the self-styled ‘Gypsy King’ said on Wednesday.
“It’s been done many different times by many different fighters. He did it that night and I don’t think he will come back from it. Finished. Ask any top trainer who has been around the sport a lifetime.
“When he got to the ring I saw he didn’t want to be there.”
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.
Joshua’s promoter Eddie Hearn has said there will be a rematch, probably in November or December, with the venue to be decided.
Fury is also scheduled for a rematch with WBC champion Deontay Wilder, with a date to be set.
The Briton, whose fight with Schwarz is the first of a multi-fight broadcast deal agreed with ESPN in February, is the only boxer Wilder has faced and not defeated.
That was after the pair battled to a draw in December in Los Angeles.
Frank Warren, Fury’s British promoter, said in January that his man was bigger box-office than Joshua.
“Everybody thought that man could beat me,” Fury said of Joshua. “In what world could he ever beat me? What were people seeing?
“Physically Joshua could out-train everyone,” he added. “He would break every heart monitor, has probably worn out every treadmill in the gym, smashes the bag, (but) it’s all very unimportant.
“The two best conditioned fighters in the last 20 years have been (Ukrainian Wladimir) Klitschko and Joshua and they both got done by two fat men.”
Fury, now 30, beat Klitschko in 2015 for the titles he then lost due to a two-and-a-half year absence from the sport following mental health issues and a failed drug test.