Ellis was shaky early for the Stars filling in for Lehtonen on the second night of a back-to-back in a 4-2 loss to Columbus on Tuesday. He is 5-6 with a 3.04 goals-against average.
The Stars added a veteran three years removed from winning the Stanley Cup as they try to end a five-year postseason drought. Dallas started the day holding the final playoff spot in the Western Conference.
Thomas had a 2.97 goals-against average in 40 starts for the Panthers after sitting out last season. He helped lead Boston to the Stanley Cup in 2011.