The team said on its Web site the extent of the injury was not fully clear, and an MRI would provide further information.
Williams was hurt when hit by cornerback Orlando Scandrick near the end of a no-pads, no-contact practice session before 26,460 fans Thursday at the team's new stadium.
"We don't know anything about Roy, so let's don't speculate," Coach Wade Phillips said immediately after practice. "I'll just wait to see."
Another wide receiver, Sam Hurd, has not practiced this week because of a thigh bruise, meaning Dallas will be without two of its top four wideouts for Saturday's pre-season contest against the San Francisco 49ers.