Hagler is predicting the welterweight fight between the world famous welterweights in Las Vegas on May 2 to be the “biggest fight in history”, regardless of who wins.
The fight has echoes of Hagler’s 1987 clash with Leonard, also staged in Las Vegas, which saw him lose on points in a decision which remains controversial to this day.
“Whether Pacquiao loses in the first round, whether he knocks out Mayweather in the first round, it’s still going to be the biggest fight in history,” said Hagler.
Mayweather, 38, has previously recalled watching the Hagler-Leonard clash.
“I remember being a kid and saying there would never be another fight bigger than this,” he said in an interview before his 2007 fight against Oscar De la Hoya.
The American, who has a 47-0 record with 26 knockouts, is homing in on the iconic 49-0 record of 1950s legend Rocky Marciano, who retired as an undefeated heavyweight champion.
Pacquiao, by contrast, holds a record of 57-5 with two draws and 38 knockouts.
The 36-year-old Philippines boxing hero earlier this month called the face-off with undefeated Mayweather “the real fight of my life”.
Hagler was undisputed world middleweight champion for seven years until Leonard returned from a three-year retirement and upset the odds at Caesar’s Palace in a clash which was dubbed ‘The Super Fight’.
Reluctant to draw parallels with the Pacquiao-Mayweather clash, he said that both fighters would just be focused on the result.
“Once you start getting yourself into the fight, it’s not about the money, it’s about the win.”