Mayweather Pacquiao Rematch Will Happen

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.

"Mayweather? If he decides to go back to boxing then that is the time we are going to call the shots," Manny said ... "I have the…