-- HOUSTON -- No. 1 overall pick Jadeveon Clowney had arthroscopic surgery on his right knee Monday and will be out four to six weeks, Texans coach Bill O'Brien said.
The surgery was to repair a torn meniscus Clowney suffered late in the second quarter of the Texans' 17-6 win over Washington, a source confirmed to ESPN's Adam Schefter.
"He's been good," O'Brien said. "He's a great kid. He's a resilient kid. He's been a good teammate since the day he walked in the door here. He was disappointed, obviously. ... We've got great doctors here and he'll be back."
This is Clowney's second surgery since the Texans drafted him first overall in May. He had surgery to repair a sports hernia in June, which caused him to miss the first few days of the Texans' training camp.
He played well upon his return and started in the Texans' preseason opener, but his health became an issue again shortly thereafter.
Clowney left a joint practice in Colorado on Aug. 20 after a helmet-to-helmet hit with Denver Broncos tight end Jacob Tamme. He missed the Texans' final two preseason games recovering from the ensuing concussion, but passed the NFL protocols for head injuries in time to play in the Texans' season opener.
During his three seasons at South Carolina, Clowney only missed two games due to injury. He'll now be the first defensive No. 1 overall pick since 1992 to miss any games his rookie year because of an injury, according to ESPN Stats and Info.
Clowney suffered Sunday's injury after jumping in the air to try to bat a pass and landing awkwardly on his right leg. He was visibly upset as he left the field and went to the locker room just before halftime of the game.
"I feel his frustration, I would be mad too, but he's just gotta bounce back from it," said Texans safety D.J. Swearinger, who also played college football with Clowney. "... He's down. Any player would be. He's gotta keep his head high. Things happen, you gotta look at it as a plus, a positive, just to sit back and watch the game. Just learn mentally. Then when he gets back in, he'll be good to go."
Whitney Mercilus will replace Clowney as the Texans' starting outside linebacker across from Brooks Reed in the Texans' base defense. Reed and Mercilus were the team's starters during the 2013 season. It was Mercilus' first season as a starter. He was selected 26th overall in the 2012 draft.
He notched six sacks his rookie year, a franchise rookie record, and seven last season.
Mercilus initially struggled to adapt to the Texans' new defensive scheme under defensive coordinator Romeo Crennel, but began improving significantly as the Texans' preseason wound down.
"It's just not thinking, being comfortable in the defense," Mercilus said. "... This year there's more responsibilities on the linebackers, pretty much understanding schemes, formations, understanding the concepts of the defense, understanding where everybody's fitting up and where we have to fit up ourselves."
The Texans defense moves players around to different positions, tailoring each player's role to his strengths, so Mercilus won't be used in exactly the same way Clowney was. O'Brien was pleased with Mercilus' performance Sunday. He also singled out Reed as having had a strong game. Reed had four tackles, one sack for a 12-yard loss and three quarterback hurries. Undrafted rookie Jason Ankrah will also be called upon to help fill the void.
"It's football," O'Brien said. "That's just the nature of the game. It's not a matter of frustration. ... Every team in the league is dealing with injuries. The next guy steps up. That's why you try to put together the most competitive roster you can."