Sakakibara posted photographs on Twitter of the 40-year-old Pacquiao signing a contract.
"I’m here in the Philippines," wrote Sakakibara.
"RIZIN.15 will have this man involved in a way. We will be ready to announce as I get back to Japan!"
He did not specify in what capacity Pacquiao would be involved in the event scheduled for April 21.
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Pacquiao's last fight was a unanimous decision victory over American Adrien Broner in January.
Mayweather, who beat Pacquiao in a drab 2015 bout, came out of retirement briefly to score an easy victory over kickboxer Tenshin Nasukawa in the main event of Rizin's prestigious New Year's Eve show.
(Reporting by Jack Tarrant; Editing by Peter Rutherford)
