Sylvester Stallone says he's almost positive Floyd Mayweather will accept Manny Pacquiao's challenge and take a rematch with the boxer in the near future ... and Sly's spelling it out for TMZ Sports.
"I think it's about 99% certain it's gonna happen," Stallone said while leaving his wife Jennifer Flavin's birthday dinner at Spago.
"They have every reason -- honor, money, history and you got nothing better to do for the weekend."
Stallone's referring to the comments Pacquiao made in July after his victory over Lucas Matthysse -- when he essentially called out Floyd.