Pacquiao, who was defeated by American Mayweather in a unanimous points decision in Las Vegas in 2015, will defend his WBO welterweight belt against former schoolteacher and rank outsider Horn in Brisbane on Sunday.
"He wants a re-match with Mayweather," Roach told Sky News of 38-year-old Pacquiao. "To get Mayweather, you have to look good. Against Horn, he's got to look good, he has to be impressive."
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Mayweather, who retired in 2015 with an unbeaten record of 49-0, has agreed to come out of retirement for a 12-round fight against mixed martial arts champion Conor McGregor.
Pacquiao has failed to win by knockout since a 12th round TKO win over Miguel Cotto in 2009 but Roach said the fighter was primed to cause a stoppage against 29-year-old Horn, who is undefeated in 17 fights against mostly obscure opponents.
"[Pacquiao] doesn't like to predict knockouts and so forth but I do," said Roach at a media conference on Wednesday. "I think it's going to be short and sweet. If our opponent comes out like he says he's going to, Manny will fire back and I think it's going to be great fight [but] it won't last too long. And someone will get knocked out."
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The 'Battle of Brisbane' at Lang Park, a 55,000-seat venue used mainly for professional rugby matches, is the biggest fight in decades 'Down Under', throwing Pacquiao's unheralded opponent Horn into a huge spotlight.
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