HOUSTON -- Houston Texans linebacker Mario Williams is out for the season after tearing a pectoral muscle in Sunday's 25-20 loss to Oakland.

Coach Gary Kubiak said Williams will go on injured reserve and undergo surgery in the next week.

Williams appeared to be hurt while sacking Jason Campbell with about five minutes left in the first quarter. He walked to the locker room on his own, and was on the sideline in the second half, wearing a T-shirt and shorts.

Williams, the top overall pick in the 2006 draft, moved from defensive end to outside linebacker this season in new coordinator Wade Phillips' 3-4 alignment. He was growing more comfortable with the role each week, and had five sacks and a forced fumble before the injury.

"I feel so bad for Mario, because he's such a heck of a player," Kubiak said Monday. "He had really taken to the position he's playing. He was improving every day in practice, improving in every game. I'd never seen him so upbeat about what was going on."

The Texans (3-2) play at Baltimore (3-1) on Sunday, and Williams isn't their only injury concern.

Quarterback Matt Schaub has a sore throwing shoulder and a thigh bruise, and fullback James Casey left Sunday's game with a strained pectoral muscle. Kubiak said both players are "day to day."

Right guard Mike Brisiel left Sunday's game with injuries to his ankle and knee, and Kubiak said he will be re-evaluated on Wednesday.

The Texans were already playing without star receiver Andre Johnson, who injured his right hamstring in last week's win over Pittsburgh. Kubiak said Johnson is no longer sore after a minor procedure last Tuesday, but he couldn't say if Johnson could be ready in time for Sunday's game against the Ravens.

"His rehab has picked up," Kubiak said, "so we'll see where he's at each day."

The loss of Williams is a major blow to a defense that had showed vast improvement from last season.