All the talk in the lead-up to the bout surrounded Ruiz weighing in at the heaviest he's been in his decade-long career - 7kg heavier than when he shocked the world back in June.
After the fight, he admitted he had gained too much weight and didn't prepare like he should have.
British boxer Anthony Joshua and his American rival Andy Ruiz Jr struggle in their bout. Photo / Getty
Parker told Radio Sport Ruiz had no grounds to be making excuses.
"He had a long time to prepare. He had his team, he had his chef, so I think he didn't keep his eyes on the prize, I think he got comfortable being unified champion of the world," Parker said.
"I don't think he took it as seriously as he did in the first fight.
"If you come from a struggled life and you come from nothing and then all of a sudden you become the unified champion of the world, you get all this money and you go off on a happy trip, maybe he went off the tracks a bit."
Parker, who lost to Joshua by unanimous decision last year, praised the Brit for his impressive performance.
Joseph Parker is staying ready to head back into the ring. Photo / Photosport
He said it's unlikely the current champ will fight Ruiz again.
"AJ, after winning this fight, will go onto bigger and better things, he just proved that he can box ... he's a great champion," said Parker.
"This is the lightest he's been in a long time and being light, being the way that he is, he was able to move around a lot more, a lot better, and he didn't tire."
Parker is currently in Samoa helping families in communities affected by the measles crisis but will return to New Zealand in the coming weeks to continue training.
He said he's staying ready for the ring.
"As much as I'm frustrated, I've been keeping training and I've been keeping fit. I'm ready to go anytime, I'll fight anyone, anywhere, anytime, I don't care who it is," Parker said.
"I've never really taken my training in New Zealand seriously until the last half of the year ... I just want to give it 100 per cent and if I become champion of the world I'll be happy, but if I don't - at least I gave it a good crack.
"I'm giving myself four years to fight hard, be the best I can and see where I get."
