Joshua surrendered his WBA, WBO and IBF heavyweight titles in a shock defeat to Ruiz Jr earlier this month.
The Brit was caught with a third-round left hook on the top of the head and was wobbled for the rest of the fight.
He hit the deck twice in that round and twice more in the seventh before the referee waved off bout.
And Fury, who is gearing up to face Tom Schwartz in Las Vegas this weekend, believes Joshua wanted to hit the canvas and in turn stop the fight.
“I saw a weakness in Joshua’s mental make-up,” Fury told talkSPORT.
“When he walked to the ring he looked like he wanted to be anywhere but in the ring that night.
“He got caught and anybody can get caught in round three.
“No shame in going down but it’s the way that the sort of accepted defeat.
“It’s the way in round seven he went down because he wanted to, not because the punches were putting him down.
“It was like he was jumping to the floor.
“That’s true sign of somebody who doesn’t want to stand and fight.”
Joshua instantly activated the rematch clause and will attempt to regain his belts later this year.
But Fury does not reckon the 28-year-old will be able to avenge his recent loss.
“I think Ruiz would win again,” he added.
“It’s how the defeat happened. It’s not the fact of losing the fight, it’s how one loses.
“The fact he didn’t want to fight on, his body language said he had enough. He spat his gum shield out. It’s all the signs he didn’t want to be there.”