No one can deny that Manny Pacquiao has been a warrior in the ring, having been in many physical pugilistic wars with the likes of Marco Antonio Barrera, Erik Morales, Miguel Cotto, Oscar De La Hoya, and Juan Manuel Marquez.

But one has to wonder how much "Pacman" has left in the tank after suffering his second straight loss at the hands of Marquez, in which Pacquiao was knocked out in devastating fashion after a beautiful counter-punch by Marquez that left him laid out on the mat.

Brandon Rios aims to find out if Pacquiao is still one of the best pound-for-pound boxers in the world in their welterweight bout taking place at the Venetian Macao Hotel in China Saturday night.

Rios, also coming off a loss after a thrilling rematch against Mike Alvarado, in what is a potential "Fight of the Year" candidate, losing by unanimous decision after beating Alvarado five months earlier by technical knockout in what Sports Illustrated named "Fight of the Year" in 2012.

Rios, who many feel is a tailor-made straight-ahead type of fighter perfect for Pacquio's comeback, is a 4-to-1 underdog who feels he has something to prove after his last defeat to Alvarado and has no intentions of laying down for Pacquio.

"I'm nobody's tune-up fight or sparring partner or heavy bag," said Rios to The Guardian. "Because a heavy bag don't punch back."

After Marquez's knockout of Pacquiao, questions linger regarding how much he has left in his storied career that has seen him fight against the best in various weight classes (Pacquio started his career in light flyweight division and is currently competing at welterweight); especially after losing the last two fights to Marquez and Timothy Bradley.

Rios believes that between the many battles against heavy punchers like Morales and Barrera as well as fighting boxers in heavier weight classes such as Cotto, Joshua Clottey and Antonio Margarito, the amount of punishment that comes with the sport has taken its toll on Pacquiao's body.

"I think it wears down your body," said Rios. "Maybe he's got that speed in training, but not when he gets to competition."

Another factor that may give Rios a legitimate shot at an upset is that Pacquiao is no longer just a boxer. Pacquiao won a seat in the Philippines House of Representatives in 2010 and now must juggle his civic duties as a politician with his boxing career. One has to wonder if Pacquiao has taken on too much in his life between the sport he loves and his future career.

And while Pacquiao's congressional district was spared the devastation of Typhoon Haiyan--which has left 673,000 displaced, injured 23,000 people, 1,611 still missing, and a death toll of 5,209--the fighter is very concerned about helping out his people in their time of need.

"We thought about going to ground zero, and we were talking to the camp about it, and we discussed it and we agreed that if anybody goes, the camp will go," said Freddie Roach, Pacquiao's longtime trainer. "That's what we did in Baguio with the typhoon up there [in 2009]. But this time the fight was getting too close, and we decided Manny winning the fight was the best thing we could do. Manny has sent a lot of money down there, a lot of food down there, he's sent his people down there, the congress people who work for him. He's done everything possible that he can do, and his goal now is to win the fight for his people."

With nothing to lose being the underdog, a chip on his shoulder, and something to prove, Rios has got to like his chances for an upset if he can stay patient and wait for Pacquiao to give him an opening much like he did against Marquez did last year.

"He's going to be dangerous for four or five rounds," said Alex Ariza, Rios' strength and conditioning coach who used to work for Pacquiao before an acrimonious split with Roach that even led to a fight between both training camps earlier in the week. "But if he starts looking in the wrong direction and starts getting flat-footed, it could be trouble."