"This guy [Horn] has nowhere near the experience at all. This f***** [Horn] hasn't walked out in front of 50,000 people, ever.
"Nerves take a lot out of you when you fight. It's actually terrible, there's nothing you can do about it, happens to all of us.
"Nerves will sap the living shit out of you. Manny's done it, he's used to it, it's no big deal. Jeff's not. I don't think they've realised that."
Fortune, who owns a gym in Hollywood and has been working with Pacquiao and Freddie Roach since 2002, also warned Horn that he had made a major error by not travelling to the United States to prepare.
"That's my big thing with Jeff. Had you gone to the US for six weeks and prepared properly in a training camp, I'd give you a hell of a shot," Fortune said.
"But he's staying in Queensland and sparring with some Filipino [Czar Amonsot]. Hopefully we can pull some more Aussie boxers out there to come to America and learn their trade, like I did, in the best place in the world.
"That's how you've got to succeed."
Fortune said he had no reservations helping Pacquiao plot the downfall of a fellow Australian.
"This event is huge for Australian boxing. It's on ESPN, millions of people will see it," he said.
"But I don't know Jeff Horn from a bar of soap. He's an opponent. We're in the hurt business. You've got to go out there and hurt them.
"Horn's got no way out. He's got to come out there and fight, and he will. There's no dog in Horn."
Horn's trainer Glenn Rushton, however, hit back yesterday saying the former school teacher would continue to embrace the high-pressured build-up to what is being billed as the biggest boxing event on Australian soil.
"I keep looking at Jeff all the time, I talk to him," Rushton said. "I only have to look at him to see if he is coping, and he is coping well. His eyes are bright and clear. He is good."AAP
