The portly Mexican-American stopped the Briton in the seventh round in New York, knocking him down four times in a huge upset.
The Londoner will attempt to regain the WBA, WBO and IBF belts he lost that night in 16 days’ time in a rematch in Diriyah, Saudi Arabia.
But Joshua says whatever sense of loss he felt when defeated by Ruiz Jr does not compare to his feeling when beaten in the final of the World Amateur Championships by Magomedrasul Majidov in Baku in 2011 because he is so much better able to cope with defeat now.
“One loss can’t strip your skin off overnight,” said Joshua. “When you put your solid foundations in, one chip in the brick can’t destroy the whole building.
“Defeat in Azerbaijan [hurt more]. I know why I lost in New York. But Azerbaijan hurt more. I don’t know why, it just did. With Ruiz there was stuff that I was going through, but it is what it is, I’ll win in December.”
The loss to Majidov burned, not helped by the widespread feeling that he should have been given the judges’ decision against the hometown boxer in a bout that took place in front of the Azerbaijan president.
It was also just three years after Joshua had first taken up the sport, hoping it might provide an alternative to his regular run-ins with the law.
“There was just that feeling of hurt when I lost at the worlds,” he said. “I walked into the gym on tag and next minute I’m fighting at the World Championships.
“I didn’t know what the World Championships were, I didn’t know what the Olympics were, I didn’t really follow sport. I liked watching all the gangsters and stuff on YouTube.
“With Ruiz I know what this is about, I took my loss and I know I can get it back. No excuse, I give Ruiz credit, he won, he beat me, he caught me with a good shot, I shouldn’t have been there [to take the punch]. I know I can come back and do it again.
“I can fight, that is it, I don’t need to reflect on that, I just need to win.”
The defeat does not seem to have dented Joshua’s enthusiasm for the sport and despite suggestions that taking the money on offer in Saudi Arabia was a cash-out fight, the 30-year-old insists he is in boxing for the long haul.
There were also calls to drop his trainer, Rob McCracken, something he refused to consider. So much of Joshua’s training set-up has remained the same, although extra help has been called in to ensure that he gets sufficient pad work.
Joshua spoke to the press at his gym at the English Institute of Sport in Sheffield yesterday, although for the first time before a big fight he did not allow the media to witness any of his training.
McCracken says that the defeat to Ruiz Jr seems to have inspired Joshua’s sparring partners, however.
He can face five different partners during a single session and they have not held back.
“Ever since Josh has been a professional, sparring partners have always come here to try to rip his head off and make a name for themselves,” McCracken said. “This group of sparring partners has done really well and pushed him every step of the way and we still have three very important spars ahead, where we are going to work on our strategy.”
Ruiz Jr v Joshua II, live on Sky Sports Box Office, December 7. Go to skysports.com/joshua