Having stepped in as a late replacement for Jarrell Miller, Ruiz shocked the boxing world when he recorded a seventh-round stoppage win over the previously undefeated Joshua in June this year.
However, the IBF, WBA and WBO champion failed to prepare properly for their second bout, weighing in at 20 stone and three pounds for his first defense of the belts.
In contrast, a slimmed-down Joshua - who had registered under 17 stone on the scales for the first time in over five years - was ready to go the distance, controlling the bout from start to finish as he recorded a unanimous points win on the scorecards.
Ruiz hopes the pair will make it a trilogy of fights by facing each other once again in 2020, as well as praising his successful opponent for doing "a hell of a job".
"It was his night. I didn't prepare how I should have, I gained too much weight," he told Sky Sports Box Office. "I don't want to give too many excuses, he boxed me around.
"[The weight] affected me. I thought I'd feel stronger, thought I'd feel better.
"I tried to train myself, but I don't want to give no excuses. Anthony Joshua did a hell of a job.
"If we do the third fight, you best believe I'm going to get in the best shape, be in the best shape of my life."
When asked if he would be willing to face Ruiz again, Joshua replied: "Without a doubt. Listen, if you heard – we are going to do a third."