The American-Mexican fighter shocked the world with a devastating seventh round TKO win over heavy favourite Anthony Joshua Sunday afternoon [NZ time] at Madison Square Garden in New York.
Parker, 27, congratulated Ruiz Jr on his victory and said the referee made the right decision in stopping the fight.
"The ref gave him [Joshua] a chance, good on Andy," Parker told AB Boxing News.
Parker is the only fighter to defeat Ruiz Jr in his professional boxing career, beating the Mexican brawler by majority decision in 2016 to win the WBO heavyweight title.