According to him, Joshua is "not good at boxing" and vowed to land another victory when they fight in a rematch.
The 29-year-old took to the streets of California, where he was born, in an open-top vehicle to celebrate winning the IBF, WBA and WBO titles with a seventh-round stoppage victory at Madison Square Garden.
Leapai has fought for a world title in the past, losing to Wladimir Klitschko back in 2014.
Win that and Ruiz says he'd like to fight Parker again.
In the aftermath, he said: 'I know his flaws.
Plenty of conspiracy theories have emerged to try and explain Joshua's unexpected loss after a lacklustre performance at Madison Square Garden.
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Ruiz who earned a reported $7million (£5.5m) for the bout told reporters: "First of all I bought a vehicle for my mum, that was the first purchase I made from the money I got".
Ruiz predicts his sequel with Joshua will produce the same result, saying he can be even better prepared than he was last time when he took the fight at short notice, replacing American drugs cheat Jarrell Miller.
Ruiz Jr shocked boxing fans and pundits when he knocked down Joshua four times on June 1. "I don't know if people want me to talk bad about him, but that's not the type of guy that I am", the unified heavyweight king said.
Meanwhile, it is rumoured AJ's rematch with Ruiz will be on November 16 - if the fight takes place at either at Wembley or Tottenham's new £1billion stadium. 'There is going to be a lot of changing, I am going to come at a better weight this time.
"It's going to be a hell of a fight - Ruiz vs Joshua II".
"It wasn't too big of a shock to me".