The British boxer will take on the winner of Tyson Fury and Deontay Wilder.
The Mexican admitted the extra weight had an impact on his performance.
The Englishman put on a clinical performance in Saudi Arabia.
The Briton reclaims the world heavyweight boxing title from the Mexican in Saudi Arabia.
But no-one's underestimating the Mexican, after his heavyweight upset in June.
The former champ will await the result of Sunday's Ruiz v Joshua title bout.
But the Kiwi trainer is predicting victory for the Brit.
"I've had a wonderful career. I'm going to sail into the sunset now."
The former heavyweight champ says victory will restore respect to his name.