Cotto (left) and Pacquiao officially announced the fight at Yankee Stadium
Manny Pacquiao insists he will be too quick for Miguel Cotto when the two men meet in Las Vegas on 14 November.
Philippine superstar Pacquiao, who knocked out Britain's Ricky Hatton in two rounds in May, said: "I know Cotto is bigger and stronger than me.
"My speed is my advantage and what's in the heart makes the difference."
"No matter how much speed he has, when you have a good defence, you can stop speed," countered Puerto Rico's Cotto, the WBO welterweight champion.
"I'm not Ricky Hatton, I'm not Oscar de la Hoya [who Pacquiao beat last December]. Everyone knows what Miguel Cotto is capable of doing.
I'm getting Manny ready for the fight of his life. We've got our work cut out for us
Freddie Roach, Manny Pacquiao's trainer
"I'm going to train the hardest in my career. I'm just trying to get myself fit enough to capitalise on any mistakes."
Pacquiao, 30, has only fought as high as welterweight once before, against American legend De la Hoya, and his victory over Hatton took place at light-welterweight.
And his trainer Freddie Roach conceded Cotto, 28, would be Pacquiao's most formidable foe yet.
"I'm getting Manny ready for the fight of his life," said Roach. "We've got our work cut out for us."
Pacquiao, who has won world titles in six different weight divisions, was at ringside in New York in June when Cotto won a decision over Joshua Clottey.
"I'm impressed. He's strong and he can punch," said Pacquiao, who has 49 wins and three defeats from 54 fights.
"I study the style of my opponent and apply some techniques to beat him. I saw some weaknesses in Cotto but I won't tell you what I saw."
Cotto's only defeat in 35 fights came last year against Mexico's Antonio Margarito, who was suspended after his trainer was found to have loaded his gloves before a January bout against Shane Mosley.
Cotto, a former world champion at 140lb, stopped Britain's Michael Jennings in five rounds to claim the vacant WBO welterweight title in February.