The Californian power-puncher came in weighing 20st 3lbs and looked slow and heavy as he lost his world titles.
Ruiz Jr was 15lbs heavier than when he beat AJ at Madison Square Garden in June and the added weight told in Saudi Arabia.
He was beaten to the punch all night by a man three stone lighter than him and was too slow on his feet to get near the Brit.
Ruiz Jr lost a wide points decision but called for a trilogy fight.
He told Sky Sports: “I didn’t prepare how I should’ve and gained too much weight. He won and outboxed me but best believe if we do a third fight I’m going to get in the best shape of my life.
“I thought I was going to feel stronger and better but next fight I admit I’m going to be more prepared. I was chasing him instead of cutting the rings. I felt like I couldn’t throw my combinations.”
All training camp, Ruiz looked to be faster and scarier than when he knocked Joshua out in New York in June.
Ruiz Jr tried to shrug it off, saying: “I was already the same weight as the last fight this morning.
“I had to wait until almost 5pm to get on the scale, so I already had my breakfast, lunch and dinner. I also weighed in with my sombrero and pants on, so that’s why I came in so heavy.”
“It was a psychological thing as well.”