MMA Weekly first reported the news, and Strikeforce director of communications Mike Afromowitz confirmed Sunday to MMA Fighting that the heavyweight is out – just one week before the event.
Valentijn (28-25) was scheduled to fight Antonio Silva on Saturday's main card on Showtime. It was to be his Strikeforce debut.
Valentijn is the second main-event fighter in less than a week to be scratched from the event. Last week, former NFL star and Heisman Trophy winner Herschel Walker suffered a cut under his eye that forced him out of his second pro bout. He was to face Scott Carson (4-1), who in June returned to MMA after a layoff of about nine years. He suffered a knockout loss in his first fight back after going 4-0 between 1999-2001.
Replacements for both Valentijn and Walker have not yet been secured.
Valentijn has won both his fights in 2010, getting back on track after a stretch that saw him drop six of his eight previous fights. He also had a stretch between 2001-03 in which he lost nine of 10 bouts. At a Pancrase event in April, he won by head kick knockout in just seven seconds. His K-1 win in May came in 40 seconds.
Valentijn's highest profile wins have come against UFC Hall of Famer Randy Couture in the Rings King of Kings 2000 tournament semifinals and Renato "Babalu" Sobral. He has twice lost to Antonio Rodrigo Nogueira and Gilbert Yvel.
Silva (14-2, 1-1 Strikeforce) lost his Strikeforce debut against Fabricio Werdum a year ago, but rebounded with a decision win over Andrei Arlovski in St. Louis in May.
Strikeforce: St. Louis takes place Saturday at the Scottrade Center and is the promotion's third event in the city. The main card is scheduled to air live on Showtime and features a main event light heavyweight bout between Dan Henderson and Sobral and a co-main event between middleweight contenders Robbie Lawler and Matt Lindland.