Yet Pacquiao was a picture of serenity and confidence as he smiled and waved to a mostly Filipino crowd attending the weigh-in on Saturday at the The Venetian casino in Macau.
The WBO international welterweight title is the official prize, but for Pacquiao this fight is about much more.
Coming off two consecutive losses — the most recent a devastating knockout at the hands of Juan-Manuel Marquez almost a year ago — Pacquiao has been written off by many as a fading star, close to his 35th birthday, and whose days as the world's pound-for-pound champion are gone for good. It's been four years since he stopped an opponent, and favourable expert comparisons with Floyd Mayweather Jr. seem like distant memories.
Long-time trainer Freddie Roach had upped the stakes by saying Pacquiao should retire if he does not perform up to expectations in Macao.
If that pressure was not enough, Pacquiao must also deal with the turmoil back in the Philippines, which is still struggling with the impact of this month's typhoon which killed thousands and made hundreds of thousands homeless.
Any ordinary fighter would struggle to prepare for a bout while his nation dealt with such a tragedy, but Pacquiao is no ordinary fighter. He has transcended the sport to take on the status of national icon, and is officially a congressman in Sarangani province.
When the typhoon hit, Pacquiao was already in training in General Santos city and it was hard for him to resist pleas to leave camp to help out and raise morale in the disaster zone.
Convinced by Roach and other trusted advisors, Pacquiao made the tough decision to stay in training camp with the aim of delivering a victory that would raise national morale.
Reminders of the typhoon surrounded him at the weigh-in, with fans waving banners referencing the storm, and others shouting out in Tagalog for Pacquiao to "Do it for Tacloban!" - the city worst hit by the storm.
"I'm doing my best to give a good fight and to win the fight, especially with what happened to my countrymen and what happened to the Philippines with the typhoon," Pacquiao said. "To all the people and families affected by this storm, this fight is for you."
Roach, who has publicly been supremely confident of victory and thereby doing his part to rebuild confidence in his fighter after the Marquez defeat, said the national disaster had genuinely stirred Pacquiao despite his outward calm.
"Winning the fight is going to do more for the country than anything in the world," Roach said. "His goal is to win the fight for them, for his people, and that's what makes Manny tick.
"I hate to say the typhoon was a good thing, I don't want to say that, it was tragic. But Manny will fight for his people and that's what's going to motivate him to win this fight."
Pacquiao will receive a reported $18 million for this pay-per-view fight, five times the purse Rios will receive, with that disparity accurately reflecting the underdog status of the American, raised in Kansas and now fighting out of the gym of Robert Garcia in Oxnard, California.
Rios too is coming off a defeat — the first of his professional career — in the rematch against Mike Alvarado but the former lightweight champion is still on the rise, aged 27, and with a height, reach and likely punching-power advantage over Pacquiao.
"They think I'm no problem but I'm going to be a huge problem," Rios said. "Everyone thinks I'm a tune-up fight; I'm nobody's tune-up fight and nobody's punching bag.
"In the past when Manny hurt you he would finish you and that could be wearing off a little bit. I'm here to test him, and I'm not going to let him forget that knockout against Marquez."
Timothy Bradley, whose highly contentious points decision over Pacquiao preceded the Marquez defeat, said the 'Pacman' would likely be more than Rios can handle.
"I've been in the ring with this guy and what's special about him is that he throws combinations and every shot is a death blow," Bradley said. "Rios is a big puncher and I know he comes to fight but he's a tad too slow for Pacquiao."
The undercard begins at 8 a.m. local time (7 p.m. ET) with the main event expected to begin around three hours later.