By Kieran Mulvaney
"Joshua Clottey is taller and bigger than me," Pacquiao told reporters in a conference call on Friday. "He is a former (IBF) champion. I cannot underestimate him."
Pacquiao and Floyd Mayweather Jr, widely regarded as the two best pound-for-pound boxers in the world, had been scheduled to fight on March 13.
However, negotiations collapsed when the Filipino refused Mayweather's demands for Olympic-style dope testing procedures.
Pacquiao filed a lawsuit against Mayweather, the American's father Floyd Sr, his uncle Roger and Golden Boy Promotions for falsely accusing him of using performance-enhancing drugs.
"I believe Floyd Mayweather was not ready to fight me," said Pacquiao (50-3-2, 38 KOs). "That's why he makes some reasons to cancel the fight.
"I feel bad that he is accusing me of doing drugs, that he tries to ruin my name in boxing. I'm disappointed for what he's accusing me of but I'm not angry at him. That's his style."
After the collapse of the Mayweather fight, Pacquiao turned to Clottey (35-3, 20 KOs).
It is a fight Pacquiao's trainer Freddie Roach is confident his man will win.
"Clottey's a very strong guy, he's a good puncher," Roach said. "I think his best punch is the uppercut.
"But I have a lot of confidence in my fighter. He's sparring with guys who fight quite a bit like Clottey.
"We have big, strong guys and Manny's handling them with ease. I think we will overwhelm him and I think the fight will end before 12 rounds are over," added Roach.
"I think Manny will be the first to knock him out."
(Editing by Tony Jimenez)