The 2017 season is over for Whitney Mercilus and J.J. Watt.

Monday morning at NRG Stadium, Texans head coach Bill O'Brien confirmed the news, as both defenders went down with injuries in Sunday night's loss to the Chiefs.

"To have their seasons cut short because of an injury, it's tough," O'Brien said. "But, and that's a big but, this football team understands that it is the 'Next man up' philosophy."

Mercilus suffered a chest injury early in the opening series against Kansas City, while Watt injured his knee later in the same possession. Just like that, two key playmakers up front were gone for the year. But O'Brien emphasized that there's no time for the Texans to sit around and feel sorry for themselves.

"The train doesn't stop moving," O'Brien said. "The train keeps going. That's the deal with the League. That's what makes the NFL what it is. You don't replace J.J. Watt or Whitney Mercilus with one guy. That's ridiculous."

Jadeveon Clowney, someone who O'Brien mentioned will continue to be a vital piece of the defense, agreed.

"It hurts, big blow to the team," O'Brien said. "A lot of our game plan is around them. There's a lot of season left. We still got some playmakers out there."

Clowney finished the game with three tackles, including one for a loss, as well as a quarterback hit.

Third-year defensive lineman Christian Covington saw his snaps rise in Week 5, as he was on the field for 58 percent of the time against Kansas City. 

"It's tough," Covington said. "Our heart goes out to them. It's time for us to step up now."

The Texans will host the Browns this Sunday at NRG Stadium.