That’s according to hall-of-fame trainer Freddie Roach.
Anthony Joshua looks set to challenge for his old belts against Ruiz later this year, either at Madison Square Garden in New York or the Principality Stadium in South Wales.
The Brit lost his WBA, WBO and IBF world heavyweight titles against underdog Ruiz in spectacular fashion earlier this month.
Joshua was dropped four times before being stopped on his feet in the seventh by the late replacement.
After the defeat, Joshua was criticised by unimpressed boxing fans and told to limit his team before the rematch.
The superstar himself stayed quiet on the fight, but confirmed that he would exercise his option for an immediate rematch.
Roach, who used to coach Ruiz, warned Joshua he has to change his approach for the return fight unless he wants to be stopped again.
Speaking to Fight Hub TV, he explained that Joshua’s impressive physique doesn’t mean anything if he doesn’t improve his defence.
“Same thing if Joshua doesn’t completely change his ways,” Roach said when asked for a prediction for the rematch.
“Ever since he fought Klitschko he’s been going downhill a little bit.
“[He’s put on] A lot more muscle, he looks really good and so forth but I think he has trouble carrying that muscle and he’s been fatigued in his last couple of fights.
“He’s been getting tired before the end and I don’t think muscles make fighters. Oh, he has big biceps so he must punch hard? Give me a break.
“I would like to say he needs a new team and so forth, and I’d like to be that part of the team, because I do like him and he’s a friend… but I’m not trying to steal a fighter.
“The thing is, I think he needs to go back to doing what he did before when he won the Olympics, because that is when he was a really, really good fighter in my opinion.”
Yesterday, Eddie Hearn revealed the rematch could take place at Tottenham’s new stadium if the fight takes place in the UK.
He also suggested that if it took place in America again, it is likely that it will be at Madison Square Garden.