Leonard, often seen as one of the best fighters ever, won world titles in five weight classes and was the undisputed champion at 147lbs. Pacquiao, on the other hand, made history as the only boxer to win world titles in eight different divisions.
Mayweather Sr didn't hold back when he gave his opinion on the Mayweather Channel on YouTube.
"Man Leonard would beat Pacquiao, he'd knock him out cold, ice cold just like how [Juan Manuel] Marquez knocked him out."
Leonard fought during the 1980s, a time many call boxing's golden generation. He shared the ring with other greats like Marvin Hagler, Thomas Hearns, and Roberto Duran.
In November 1980, Leonard beat Duran in their rematch when Duran quit in the 8th round. He went on to defeat Hearns in 1981 and Hagler in 1987. Leonard capped off the decade by winning the WBC super-middleweight title against Duran in their third fight in 1989.
Pacquiao, now 46, isn't done yet. He'll fight for the WBC welterweight title against Mario Barrios in July. The Filipino great wants to add another chapter to his already impressive career.
While we'll never know how a fight between Leonard and Pacquiao would really go, Mayweather Sr's bold prediction will surely keep the debate going among boxing fans.