Justin Fortune, the strength and conditioning coach of WBO welterweight champion Manny Pacquiao, has issued a harsh warning to 2012 Australian Olympian Jeff Horn.

Horn (16-0-1, 11 KOs), who is a massive underdog, will challenge the Filipino superstar on Saturday night [Sunday] in Australia, before a huge crowd of 50,000-plus at the Suncorp Stadium in Brisbane, Australia.

Fortune, a former heavyweight fighter and also Australian, is giving Horn absolutely no shot of winning.

Horn has never faced anyone close to the level of Pacquiao. His biggest wins came against faded former world champion Randall Bailey and Ali Funeka, who were both several years past their best - and Horn had to get off the floor against them to win.

If he gets dropped by Pacquiao, Fortune has warned him to stay down or risk the possibility of getting a bad beating.

"He knocks you down and you get back up, he will really f*** you," Fortune advised ahead of Sunday's WBO welterweight title fight at Suncorp Stadium. "He's got that killer in him. If he sees you hurt, then you are f***ed. Manny beats Triple A fighters. This guy Horn doesn't. He doesn't have that f***ing backing. He can't pull it off. I don't think he can even reach down to his f***ing boots and pull it up. He's been knocked down a few times, he's gotten up – great – but they are against much lesser opponents," Fortune told Fox Sports.

Pacquiao, while nearing the end of his career and nearly 40-years-old, is still close to the top of his game. Last year he pulled in dominating twelve round decision wins over Timothy Bradley and Jessie Vargas. Both Bradley and Vargas would likely be favored to beat Horn - which explains just how much of an underdog the Aussie fighter really is.

The fight will be televised on free TV in the United States, with ESPN picking it up.