Promoter Bob Arum told The Associated Press on Tuesday night that the deal will be done after Mosley's lawyers approve the final paperwork for the WBO welterweight title fight at the MGM Grand Garden Arena.
Arum says Pacquiao chose the bout because Mosley is the most accomplished and famous boxer in the field of candidates, which included Juan Manuel Marquez and welterweight champion Andre Berto.
"He's the most recognizable name, and it's a difficult fight for him," said Arum, who met with Pacquiao in the Philippines last week. "People are also saying, 'Well, why hasn't he fought an African-American fighter?' He's doing that, but mostly it's because Mosley is the biggest name."
Pacquiao also realizes a potential superbout with Floyd Mayweather Jr. is likely still impossible because of the Mayweather family's various legal troubles.
Pacquiao (52-3-2, 38 KOs) is on top of the boxing world after trouncing Antonio Margarito in a one-sided decision in Texas on Nov. 13. The 32-year-old Filipino congressman has won 13 straight fights while claiming world titles in an astonishing eight weight classes, most recently taking the WBC 154-pound belt with his win over Margarito.
Arum realizes he could have trouble selling a fight against a nearly 40-year-old opponent who will be nearly 2½ years removed from his last victory when he faces Pacquiao, but he believes Mosley's skill and speed will challenge Pacquiao.
"That was obviously a negative, but Shane Mosley is capable of doing great things," Arum said.
Mosley (46-6-1, 39 KOs) is near the tail end of a remarkable career that began in 1993. The former three-division champion has won just two of his past five fights, getting walloped by Mayweather in an unanimous decision last May before fighting to a dull draw with Sergio Mora on Sept. 18.
The deal further cements Mosley's acrimonious split from Golden Boy, Oscar De La Hoya's promotional company.
Mosley was a founding partner in Golden Boy, but he has cut ties with the organization since his draw with Mora last September, even refusing to answer phone calls from Golden Boy CEO Richard Schaefer. Mosley apparently was angry that Golden Boy pushed Marquez, the talented Mexican champion, as Pacquiao's next opponent.
Golden Boy and Top Rank, Arum's promotion, are involved in a bitter rivalry. The companies have become unwilling to match any of their fighters against each other, which likely proved to be an insurmountable obstacle for Golden Boy's Marquez.
Pacquiao and Marquez have fought twice before, with a draw in 2004 and a split-decision win for Pacquiao in 2008. Both fights were thrilling contests that easily could have been decided for Marquez, who has been campaigning for a third bout since moments after their second fight ended.
After straying to suburban Dallas for his victories this year over Joshua Clottey and Margarito in cavernous Cowboys Stadium, Pacquiao is eager to return to Las Vegas, where his fights always attract thousands of fans eager to pump gambling money into Nevada's struggling gaming industry.
"It's great to do a promotion where I can sleep in my own bed," Arum said.
Mosley didn't immediately return a call to his cell phone.