Williams appeared to be hurt while sacking Jason Campbell with about five minutes left in the first quarter of Houston's 25-20 loss to Oakland.
He was taken to the locker room and did not return. Coach Gary Kubiak offered no specifics to Williams' condition after the game.
"I'm not a doctor, so I don't know," Kubiak said. "I hate to answer that right now in the state of mind I'm in. So let's see what happens. I'll get back with you when I find out."
Williams, the top overall pick in the 2006 draft, moved from defensive end to linebacker this season in Houston's new 3-4 defense. He has five sacks and a forced fumble this season.
"He said he felt like something popped, but I don't know," defensive end Antonio Smith said. "The only thing that concerned me was that he didn't come back."
The 6-foot-6, 285-pound Williams has led the Texans in sacks in each of his five seasons. He was slowed by a sports hernia late last year and underwent offseason surgery.
Houston is already playing with star receiver Andre Johnson, who sat out Sunday's game with a right hamstring injury.
"You're never going to replace a guy like Mario," linebacker Brian Cushing said. "At the same time, it's the 'next guy up' attitude. When one guy goes down, it's the same with Andre, the offense kind of needs to step up. And when one guy goes down, like Mario, the defense needs to step up."
Williams' injury created more playing time for rookie Brooks Reed, a second-round draft pick. Reed had four tackles in Sunday's game.
"Initially, I've got to say that he looked pretty impressive," Kubiak said. "Looked like he had some good rushes off the edge. He went from playing 10-12 plays, to all of a sudden, he probably played 50 in the game.
"It's gonna be time for him to step up now," Kubiak said, "but that's what he's here for."
The Texans (3-2) play at Baltimore (3-1) on Sunday, and they were fearing the worst about Williams.
"We have to pick it up," Smith said. "Mario is who he is, but somebody is going to have to come and pick up the slack. That's just the bottom line. We don't have time to dwell on whether there is a drop-off or not. We are just going to have to focus on everybody picking up the slack for Mario if he is gone."