Texans defensive end J.J. Watt suffered a tibial plateau fracture in his left leg on Sunday on the Kansas City Chiefs' first drive of the game. He is out indefinitely.

Watt will undergo further testing on the leg. His recovery timetable is uncertain, sources told ESPN's Adam Schefter.

Texans outside linebacker Whitney Mercilus tore a pectoral muscle on the same drive, a source confirmed to ESPN, and he will miss the remainder of the season. The Houston Chronicle first reported Mercilus' injury.

Watt fell to the ground while engaged in a block in the Texans' 42-26 loss. The Texans later announced the diagnosis.

"I don't have a ton of information about it, but I feel terrible for the guy," Texans coach Bill O'Brien said of Watt. "But with that being said, just knowing him and knowing the type of guy that he is, he's an amazing human being. And he will work extremely hard to be back to play for this football team. I know that."

Dallas Cowboys middle linebacker Anthony Hitchens suffered a tibial plateau fracture in August and was given a recovery timetable of eight weeks. Dallas receiver Dez Bryant suffered a tibial plateau fracture last season and missed three games.

Watt was seen exiting the X-ray room on crutches before he left the stadium in an ambulance.

He was helped to the sideline by the Texans' training staff and was looked at in the sideline tent before being taken off on a cart.

On Monday morning, Watt tweeted to apologize to his teammates and the city of Houston.

I can't sugarcoat it, I am devastated.

All I want to do is be out there on that field for my teammates and this city.

I'm sorry. — JJ Watt (@JJWatt) October 9, 2017

Watt's younger brother T.J. Watt, a linebacker for the Pittsburgh Steelers, tweeted his support.

The three-time NFL Defensive Player of the Year missed the last 13 games of the 2016 regular season because of a back injury. He played in his first regular-season game since his September 2016 back surgery in this season's opener against the Jacksonville Jaguars. In four games, Watt had 13 tackles, but he did not have a sack.

"That's tough [to lose Watt and Mercilus]," O'Brien said. "But at the end of the day, that's not an excuse. We have to do better in all three phases. I thought we competed until the end, and the next guys are going to have to step up and play.

"Guys get hurt, and the next guy in the team meeting room at that position needs to step up."

ESPN's Sarah Barshop contributed to this report.