Manny Pacquiao's legendary trainer Freddie Roach says he will encourage his famous charge to retire if he loses to unheralded Australian Jeff Horn in Sunday's world title fight in Brisbane.

Roach said the 11-time world champion had hoped to raise his profile with this weekend's WBO welterweight title bout in a bid to earn a re-match with Floyd Mayweather.

But Roach - who has trained 49 world champions - said the Filipino great would even consider walking away from the sport if he didn't dominate the undefeated Horn.

Asked what he would do if Horn won, Roach said: "Depending on how he lost but I would probably encourage him to retire.

"We have to win this fight. It is a crucial fight for us. We need to look good in this fight to get the bigger names interested.

"I expect Manny's best in this fight. I am looking for a big win here. We want to go on to bigger and better things."

The pinnacle would be another shot at Mayweather after the American emerged triumphant in 2015's so called "Fight of the Century".

"Jeff could derail us really badly. Manny is in a must win situation, he wants a rematch with Mayweather," Roach said.

"And with Mayweather you have to look good against an opponent, you have to be impressive. If Manny wins and struggles we might have to think about calling it a day. We are getting close to the end I know. If he struggles in this fight it could be over."

Pacquiao spends much of his time as a prominent Philippines senator and briefly retired from boxing last year.

But Roach warned the Filipino veteran had shown signs of "the old Manny" in Brisbane, knocking out two sparring partners - the first time Pacquiao had done that while training for a title fight in six years.

Pacquiao (59-6-2, 38 knockouts) is an overwhelming favourite against Horn (16-0-1 record, 11 knockouts) despite not stopping an opponent since his 12th round TKO of Miguel Cotto in 2009.

"I have been waiting for a knockout for a long time now," Roach laughed. "But he has shown signs of the old Manny at training."