Roy Oswalt will start in Dempster's place on Saturday against the Blue Jays.
Dempster has been placed on the MLB restricted list, and the Rangers recalled outfielder Leonys Martin from Triple-A Round Rock to take the right-hander's place on the roster this weekend.
Dempster is 1-1 with an 8.31 ERA in three starts for Texas since being acquired from the Chicago Cubs just minutes before the July 31 trade deadline. He was 5-5 with a 2.25 ERA in 16 starts with the Cubs.
Oswalt, signed in late June, was in the rotation until Dempster's arrival. Oswalt has been pitching out of the bullpen since then and is 4-2 with a 6.53 ERA in nine games (six starts).
Martin is batting .362 with 10 home runs and 38 RBIs in 45 games with Round Rock this season. He was with the Rangers for about six weeks earlier this season and batted .189 with five RBIs in 15 games.