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Patric Schneider, Associated Press
Houston Texans' Jadeveon Clowney (90) leaves with a knee injury during the second quarter of an NFL football game against the Washington Redskins, Sunday, Sept. 7, 2014, in Houston.

HOUSTON — No. 1 overall pick Jadeveon Clowney had arthroscopic surgery on his right knee Monday and will be out 4-6 weeks.

Coach Bill O'Brien said he didn't know exactly what was repaired in Clowney's knee.

Clowney was injured late in the second quarter in Sunday's win over the Washington Redskins. He has struggled to stay on the field since joining the Texans.

He left a dual practice Aug. 20 after a helmet-to-helmet hit with Denver Broncos tight end Jacob Tamme. He missed the Texans' final two preseason games, but said he passed the NFL protocols for head injuries in just over a week. He had to sit out the beginning of camp, too, after offseason surgery to repair a sports hernia.