Pacquiao, whose record stands at 50-3, with two drawn and 38 knockouts, will defend his WBO title against Clottey (35-3 with 21 knockouts) on the same date he was expected to face unbeaten US star Mayweather.
"It's going to be a good fight," Pacquiao said. "Clottey is a good fighter, a top fighter, and he is bigger than me. I have to prepare hard for this fight and train hard and focus."
Mayweather's request for unprecedented random blood testing was rejected by Pacquiao's camp to deny boxing fans a long-awaited showdown between the sport's pound-for-pound kings. It could have been a $40 million payday for each.
But Clottey has served notice that he intends to be more than a fill-in fighter destined to fall to the powerhouse from the Philippines.
"This is going to be a hell of a fight," Clottey said. "I love Manny Pacquiao. If you throw (punches) at him, he will throw back at you. I know what I'm going to face."
The matchup will headline the first fight card at $1.2-billion Cowboys Stadium, home of the famed American football squad from Dallas. Each fighter sported a Cowboys jersey as cheerleaders screamed beneath a huge video screen.
"It's a building for great competitions. It bodes well for us to get Manny Pacquiao into the ring," said Cowboys owner Jerry Jones, whose guarantee of $25 million was not enough to bring a Pacquiao-Mayweather fight together.
Freddie Roach, Pacquiao's trainer, made it clear after the news conference that hard feelings remained over the failure to make Pacquiao-Mayweather happen after a compromise of no blood test within two weeks of the fight.
Pacquiao is "disappointed the fight is not here," Roach said. "Life goes on without Mayweather. He wouldn't agree to (a limit of) 14 days before the fight. We did our best to appease him but he wouldn't go along with it.
"Mayweather's people agreed to it but Mayweather wouldn't go along with it. I don't think he wants to fight Manny. It was a smoke screen for him to say no.
"I want to beat this guy's ass, believe me, but he won't give us the opportunity. We will fight him any day of the week. But he doesn't make the rules."
Pacquiao dislikes having blood drawn so near a fight, saying it weakens him. Roach said he could not afford a blood test to distract "Pac-Man" so near a fight and dismissed the idea his fighter had anything to hide.
"There are always doubters out there but Manny Pacquiao is clean," Roach said. "He has passed every drug test in the world."