Southworth (9-5) successfully defended his belt at the Strikeforce "Melendez vs. Thomson" event on June 27. Southworth fought cautiously to secure the five-round unanimous decision win over Anthony Ruiz, whom Southworth previously lost to in a non-title bout.
Sobral (30-7) will make his Strikeforce debut after a unanimous decision win over Mike Whitehead at Affliction: Banned on July 19.
"He is the protector and I am the hunter," Sobral said. "I am already in shape and prepared to take it from him. I want to be the new Strikeforce world champion and, if I could become champion while fighting at a venue like the Playboy Mansion, that would make a good win even better."
Josh "The Punk" Thomson, who recently captured the Strikeforce lightweight title from Gilbert Melendez, will compete on this card and may defend his title if the promotion can find him a worthy opponent.