The Houston Texans, now standing at a head-scratching 3-2, may find themselves struggling to finish on the defensive end as much as the offensive end now.

Linebacker Mario Williams has a torn pectoral muscle that will keep him out indefinitely, possibly for the remainder of the season. Williams is undergoing more tests Monday, including an MRI, to determine the extent of the injury and see if there's any hope the Texans could get him back later this season.

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 on Sunday.

He was taken to the locker room and did not return. Coach Gary Kubiak wasn't specific about 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 number one overall pick in the 2006 draft from NC State, moved from defensive end to linebacker this season in Houston's new 3-4 defense. He already had five sacks and a forced fumble this season. As a result, his career numbers stand at 241 tackles, 53 sacks, and 11 forced fumbles in just six seasons in the NFL.

The 6-foot-6, 285-pound Williams has led the Texans in sacks in each of his five seasons, including 14 in 2007, the Texans single-season sack record. He was slowed by a sports hernia late last year and underwent offseason surgery. He ended the season with only 11 tackles and one forced fumble, but still had five sacks.

Houston is already playing without star receiver Andre Johnson, who sat out Sunday's game with a right hamstring injury. Johnson is hopeful he can be back from his minor hamstring surgery after only missing two weeks. That would mean he'd play against the Titans on Oct. 23.

If Williams is lost for a majority of games or, in the worst scenario, for the season, the Texans would be in huge trouble. The hybrid's backup, Brooks Reed, can do well, but Williams is a singular pass-rushing force.

The Texans next play at Baltimore (3-1) this Sunday.