But the 35-year-old Bonnar believes the stars may be aligning in his favour as he looks to pull off the monumental upset over Silva, who is moving up a weight class to fight at 205 pounds.
"I'm a little superstitious, and it's on the 13th," Bonnar told a news conference Thursday. "There is a lot of stuff with the number 13 going on, and that's a sign for me, for one. I've got a kid coming, and that to me means I need to do something spectacular. And three, I feel like I'm in a 'Rocky' movie.
"I'm this huge underdog in his backyard."
The bookies have Silva, seen as the best pound-for-pound MMA fighter on the planet, as a 10-1 to 12-1 favourite.
Bonnar (17-7 including 8-6 in the UFC) and Silva (30-4, 15-0 in the UFC) meet in Saturday's main event at Rio de Janeiro's HSBC Arena. The two were booked for the card only after a planned main event between featherweight champion Jose Aldo and Frankie Edgar was scrapped due to injury.
The six-foot-four Bonnar was semi-retired before the booking, as he was holding out for a fight with one of the sport's biggest names rather than simply looking to stay active in the UFC's 205-pound division.
As it turns out, he landed the biggest name in MMA, and despite the overwhelming odds stacked against him, Bonnar said he realizes what's at stake.
"This is a huge opportunity," Bonnar said. "I'm not a guy that always has the best luck, but once in a while a big opportunity comes along, and I try to take advantage of it."
Meanwhile, the 37-year-old Silva fights in his native Brazil for the first time since a July victory over archrival Chael Sonnen. That fight was expected to take place at a Brazilian soccer stadium before logistical issues forced the event to be moved to Las Vegas.
The six-foot-two Silva said he now wants to put on a memorable show to make up for the missed summer date.
"I want to be able to pay back Brazil the fight that I owe them to have here, so I'm really very pleased to fight with a guy like him, and I hope we can give you all a great show," said Silva. "I hope this fight will always be standing because it would be a great test for me because he's a guy who's never been knocked out, and he's a great striker."
In the night's co-main event, Brazilian legend Antonio Rodrigo (Minotauro) Nogueira (33-7-1) meets mercurial heavyweight Dave Herman (21-4). It's Nogueira's first appearance since suffering a broken arm in a December 2011 loss to Frank Mir.
Nogueira was originally scheduled to fight at July's UFC 147 event, but he withdrew from a planned appearance when his arm wasn't healing as quickly as expected. There was some concern the 36-year-old former UFC champ might not ever return from the injury, but "Big Nog" made it clear he's recovered 100 per cent and is honoured to fight on the same card as his longtime training partner Silva.
"I'll only retire when he stops," Nogueira said.
"That's going to take some time," Silva replied.
Additionally, Brazilian sluggers Fabio Maldonado (18-5) and Glover Teixeira (18-2) clash in a highly anticipated light-heavyweight matchup.
Teixeira has earned some notice as a possible challenger to UFC light heavyweight Jon Jones' title, but he'll have to get past his hard-hitting countryman before he can even consider the possibility.
"He's going to fight the best Fabio Maldonado that exists," Maldonado said. "Glover won't be fighting Jon Jones because I'm going to stop him before that."