HOUSTON — Jadeveon Clowney, the No. 1 pick by the Texans in this year’s draft, will be out four to six weeks after having arthroscopic surgery on his right knee Monday.

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

“I’m not a doctor, so I don’t know,” he said. “It’s just something they went in there and tried to fix it. It seems like it went well.”

Clowney, who had one tackle for a loss, was injured late in the second quarter in Sunday’s win against the Washington Redskins. He was hurt when he jumped up to try and bat down a pass. He put his hand to his right knee and began limping after he landed.

Some have speculated that his foot slipped between one of the 8-by-8-foot palettes of grass that make up Houston’s field. If that was the case, O’Brien said he didn’t know about it.

“He didn’t say anything to me about that,” he said. “From looking at all the angles on film, he just came down awkwardly. Most of those types of injuries are non-contact.”

Some of the Texans talked to Clowney on Sunday night and said they felt for the rookie.

“That’s just something you don’t want to happen as a rookie, as anybody really,” safety D.J. Swearinger said. “But as a rookie coming in having a lot of the hype that he has, that’s something he doesn’t want to happen. I feel his frustration.”

This isn’t the first problem for Clowney, who 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.

He’ll be replaced by Whitney Mercilus, who started each game last season and had 47 tackles and seven sacks. O’Brien was impressed with his play after he took over Sunday after Clowney was injured.

“He got pressure on the quarterback, played the run well and he just has to continue to get better and better,” O’Brien said. “And I think he’ll do that.”