ESPN.com first reported the new bout, and Strikeforce director of communications Mike Afromowitz confirmed the news with MMA Fighting Tuesday afternoon.
An open slot was made available on the Showtime telecast after Herschel Walker was forced to pull out of his fight against Scott Carson last week due to a cut he suffered in training.
Radach (19-5) hasn't competed since April 2009. The nine-year MMA veteran has dealt with a whole host of injuries since losing to Scott Smith in his Strikeforce debut 19 months ago. He typically fights at middleweight, but given the fact that this fight was put together on less than one week's notice, it will be contested at light heavyweight.
St. Preux (8-4) fought just 11 days ago, defeating Antwain Britt via unanimous decision at Strikeforce Challengers 12. The former University of Tennessee middle linebacker has won his last five fights in a row.
Here's the completed televised portion of Saturday night's Strikeforce card, which will air on Showtime from the Scottrade Center.
Dan Henderson vs. Renato "Babalu" Sobral
Paul Daley vs. Scott Smith
Matt Lindland vs. Robbie Lawler
Antonio Silva vs. Mike Kyle
Benji Radach vs. Ovince St. Preux