The Mavs hoped that Williams would be able to play Sunday against the Sacramento Kings after sitting out Friday's loss to the Golden State Warriors, but his abdominal strain did not respond well when Williams attempted to practice.
Williams was sent back to Dallas for further evaluation. He said the injury has been an issue for a couple of weeks and was aggravated in Wednesday's loss in Portland.
"It hasn't let up since," Williams told reporters. "I tried to do a little bit today in practice, but it was pretty painful."
The Mavs, who at 35-37 are fighting for one of the West's final few playoff spots, are now without their second- and third-leading scorers. Forward Chandler Parsons is out the rest of the season after undergoing arthroscopic surgery to address a torn meniscus in his right knee.
"It's definitely frustrating right now. We need every game we can get," Williams said. "It's no time to be sitting out. If I didn't feel like I'd be hurting myself and my team by being out there, I'd definitely want to be out there."