The Canadian native, who was acquired from the Cubs on July 31, is expected to rejoin the Rangers on Monday in Texas.
Dempster, who went 5-5 with a 2.25 ERA in 16 starts with the Cubs, is 1-1 with an 8.31 ERA in three starts for the Rangers.
Roy Oswalt will start in Dempster's place, something manager Ron Washington said should be viewed as a spot start.
Oswalt hasn't pitched since throwing 1 1/3 innings against the Red Sox on Aug. 8 and last started July 30, allowing eight earned runs in a loss to the Angels.
The 34-year-old veteran believes he's stretched out enough to start after throwing approximately 40 pitches in a bullpen session earlier in the week. He also threw on Friday.
Oswalt admitted that it has been a difficult transition coming out of the bullpen after starting his entire Major League career.
"It's a little different than what I'm [used] to -- usually I start every five days," said Oswalt, who is 4-2 with a 6.53 ERA over 40 innings. "Just sitting down there [in the bullpen] waiting on a phone call."
Oswalt was first informed Thursday that there was a chance he could be making the start.
Washington would not divulge how many pitches he is targeting for Oswalt, but it is expected he will be on some sort of restricted count.
Oswalt isn't sure what he will be able to give the Rangers either.
"It should be interesting when I get out there," Oswalt said.
To fill Dempster's spot on the active roster, the Rangers recalled outfielder Leonys Martin from Triple-A Round Rock of the Pacific Coast League. Martin was in uniform for Friday's series opener in Toronto.
Martin, 24, has appeared in 15 games for the Rangers this season, batting .189 with five RBIs. In 45 games with the Round Rock Express, Martin was hitting .362 with 10 homers and 38 RBIs.
"I wanted to make sure we had an extra position player body around," said Washington.
Chris Toman is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.