The quarterback got an MRI on Friday that revealed the sprain.
Against St. Louis, Stanton was replaced by Ryan Lindley instead of Logan Thomas, who was used in relief in the Cardinals' Week 5 loss to the Denver Broncos after injuries to then-starter Carson Palmer and Stanton.
If Stanton is unable to return next week, the Cardinals will be forced to use their third different starting quarterback this season. Stanton first replaced Palmer for three games earlier this year when the latter had a nerve injury and became the full-time starter when Palmer tore his ACL in Arizona's Nov. 9 game against the Rams.
Stanton had completed 54.5 percent of his passes this season for 1,602 yards, seven touchdowns and five interceptions before Thursday's game.
Lindley, 25, hadn't appeared in a regular season game since his rookie year of 2012. He was originally drafted by the Cardinals but was cut by the team in August. He returned after a stint on the San Diego Chargers' practice squad when Palmer tore his ACL.
Thomas, a fourth-round pick of the Cardinals in this year's NFL draft, completed just one of his eight pass attempts against the Broncos in Week 5 but the play went for an 81-yard touchdown.
- Ben Estes and Molly Geary