The Rangers said Friday that Dempster would be placed on the restricted list. The team didn't have any further explanation for his absence and said he was expected to rejoin the team Monday.
Rookie outfielder Leonys Martin (mar-TEEN') was recalled from Triple-A Round Rock to take Dempster's spot on the roster.
Oswalt will start in Dempster's place Saturday. He was bumped from the rotation when the Rangers acquired Dempster from the Chicago Cubs at last month's trade deadline. He is 4-2 with a 6.53 ERA, appearing in relief three times after making six starts.