Thomas was blitzing Chicago quarterback Jay Cutler when he got his leg twisted in the New Meadowlands Stadium turf and was run into by teammate Jason Pierre-Paul.
Thomas, in his fourth year out of USC, was being counted on by the Giants as they hoped to improve their secondary play from a season ago.
The Giants have already lost first-round draft pick Prince Amukamara for several weeks to a broken foot and reserve cornerback Bruce Johnson for the season with a torn Achilles Tendon.
Without Thomas, oft-injured Aaron Ross will have to take over as the starter opposite Corey Webster. The third cornerback would likely be Brian Witherspoon, a journeyman cornerback/kick returners who has been impressive thus far in the preseason.