An MRI exam Monday confirmed the torn ACL, multiple media reports said, adding that Webb is done for the year.
Webb banged his knee in a first-quarter collision with Cowboys wide receiver Dez Bryant. Webb remained prone on the field for several minutes before being helped to the sideline.
The team originally labeled the injury a sprain, but after Baltimore's 31-29 victory, coach John Harbaugh painted a potentially darker picture.
"Lardarius Webb does not look good right now," Harbaugh said. "It's a potential ACL. We've got a problem there. We don't know for sure, but that's what it looks like right now."
Added Ravens safety Bernard Pollard: "That's what happens in this game. You hate to see a guy like that go down because he's worked his tail off. Our prayers are with him to get a speedy recovery, but the next man has to step up."
Now in his fourth season, Webb has one interception, one forced fumble and a fumble recovery in six games this year.
Those weren't the only injuries for the Ravens.
They lost middle linebacker Ray Lewis to a triceps injury. Lewis will have an MRI on Monday, a team source confirmed to ESPN's Ed Werder.
And defensive tackle Haloti Ngata tore his MCL but is still likely to play Sunday against Houston, a league source told Schefter.
Information from ESPN NFL Insider Adam Schefter, ESPN's Ed Werder and The Associated Press was used in this report.