The move Wednesday means Nilstorp will back up Richard Bachman, who didn't allow a goal after replacing Lehtonen in Tuesday's 5-1 win over Los Angeles.
Lehtonen, who missed five games earlier in the season with a groin injury, is listed as day to day.
The Stars, making a late surge to stay in the Western Conference playoff race, play at Nashville Friday.
Nilstorp is 18-11-6 with the Texas Stars and ranks 10th in the AHL with a 2.26 goals-against average. He has four shutouts and a .909 save percentage. He is 1-3-1 with Dallas this season.