Sarkisian added that he will reevaluate the competition for Texas' permanent starting job after Week 1.
"Neither guy's ever started a game in their career, so we'll look at it after one week," Sarkisian said Monday. "But the good part for both those guys is I think we're improving as an offense."
Sarkisian declined to say if Card had secured the job permanently.
"He'll be starting against Louisiana," Sarkisian said. "Casey's gonna play. When, how much, I'm not exactly certain. I think there's got to be a feel to the game. Hudson's earned the right to be the starter, but somewhere in this game, Casey's gonna get his opportunities as well."
Card, a 6-foot-2, 200-pound Austin native from Lake Travis High School, was No. 40 in the 2020 ESPN300. He beat out Thompson, a junior who had a breakout four-touchdown performance off the bench in relief of an injured Sam Ehlinger in last season's Alamo Bowl win over Colorado.
Card has attempted just three passes in two appearances at Texas.
The race for the starting job was tight, with Sarkisian initially expressing displeasure at both quarterbacks' mistakes in the Longhorns' first scrimmage. After the second scrimmage on Aug. 21, Sarkisian said he was encouraged by their progress.
"Hudson's a very talented young man he's got great work ethic, he really tries to operate the way we want to operate," Sarkisian said. "As I told Casey, it's really not a knock on Casey. Both guys really started to perform very well after that first scrimmage, and it was a tough call."
The 21st-ranked Longhorns open at home Saturday against the Ragin' Cajuns in their first opener as a ranked team against a ranked opponent since 1984, when the No. 4 Texas beat No. 11 Auburn 35-27.