Yet Pacquiao is the man with momentum and he could rewrite the history books by becoming the only prize fighter in history to claim seven world titles in seven weight divisions if he defeats Cotto.
Cotto, who has 34 wins (27 KOs) and just one defeat to his name, is beloved by the Puerto Rican nation and as much a symbol of machismo and humility as Pacquiao is in the Philippines.
He faces the man who is regarded as the best pound-for-pound boxer on the planet in the wake of the Filipino's eight-round demolition of Oscar de la Hoya 11 months ago and the second-round flattening of Britain's Ricky Hatton in May.
Pacquiao, who has won 49 fights (37 KOs) and lost three, appears unstoppable. What has been startling is Pacquiao's ability to carry his power up the divisions. Both De La Hoya and Hatton were bigger men, yet De La Hoya at welterweight and Hatton at light-welterweight were dismantled by the speed of the tenacious southpaw.
Cotto is a slow starter, and if he cannot find a way to be the aggressor in a fight, he sits back and counter-punches. He also sets clever traps for opponents and although his handspeed and movement are slower than Pacquiao's, he will be dangerous in later rounds.
Pacquiao's trainer Freddie Roach said: "Manny is moving up a weight class but I am very confident in my guy and we are 100 per cent ready for this fight. I feel that he is going to knock Cotto out."
Cotto is unperturbed. "What they say and what they do does not concern me," he said. "I'm going home with the belt."
Pacquiao said: "This is the most important fight of my career. If I win, it will be history for boxing and for the Philippines."