Mike Chiappetta • E-mail

Strikeforce light-heavyweight champion Bobby Southworth has been forced to withdraw from his scheduled Sept. 20 title match against Renato “Babalu” Sobral after suffering a knee injury in training, Strikeforce spokesman Mike Afromowitz told NBCSports.com

The extent of the injury will not be known until Southworth has an MRI likely early next week, but it is severe enough that Southworth is unable to train at all.

“It could be a torn ACL, but for right now, they’re saying it’s a strain,” Afromowitz said.

Southworth will be replaced at the “Strikeforce at the Mansion II” show by Anthony Ruiz, who in his last match defeated Jeremy Freitag via unanimous decision at last month’s EliteXC: Unfinished Business show.

Depending on the severity of Southworth’s injury, the bout could become an interim title bout, but a decision on that has yet to be made.

“I think we’ll have to figure out the extent of the injury and go from there,” Afromowitz said. “It’s definitely something we’ll talk about. It’s too bad, but at least Babalu will still fight.”

The 38-year-old Southworth won the title in December 2006 with a unanimous decision over Vernon “Tiger” White. His most recent defense was a five-round decision over Ruiz in June.

Sobral is a highly regarded veteran of the UFC and Affliction promotions and will be making his Strikeforce debut. He has three straight wins, most recently defeating Mike Whitehead in the inaugural Affliction show.