HOUSTON -- Coming off two games in which Deshaun Watson and the Houston Texans' offense looked poised to make the team a standout in the AFC South, the defense struggled and Houston lost its two most consistent pass-rushers -- including J.J. Watt -- in Sunday night’s 42-34 loss to the Kansas City Chiefs.

The Texans lost Watt (tibial plateau fracture to his left leg) and outside linebacker Whitney Mercilus (chest) on the opening drive of the game. Both players were almost immediately ruled out for the rest of the game, and Watt was taken to the hospital in an ambulance. Watt injured his leg after he fell to the ground while blocking on a play. He was helped to the sideline by the Texans’ training staff and looked at in the sideline tent before he was taken off on a cart.

It’s still unknown how long Watt will be out. The Texans played without Watt for most of the 2016 season, after he aggravated his back injury in Week 3 and needed season-ending surgery. The unit, led by Mercilus and defensive end Jadeveon Clowney, finished last season ranked No. 1.

Without Watt and Mercilus, the Texans’ defense had a difficult time slowing Chiefs quarterback Alex Smith, who finished 29-of-37 for 324 yards and three touchdowns. He did not throw an interception and had a passer rating of 130.2. Tight end Travis Kelce -- Smith's top target on Sunday -- had eight catches for 98 yards.

"[Smith is] really good at extending plays," Texans head coach Bill O'Brien said. "He got outside the pocket I thought. That was one of the keys to the game: We had to try to keep him in the pocket and obviously we didn't accomplish that."

But even though Watson and the offense kept the Texans in the game -- including a fourth-quarter, 48-yard touchdown pass to wide receiver Will Fuller to get the Texans within 26-20 -- Houston’s defense couldn’t make stops when they needed to.

After Watson threw an 8-yard touchdown to wide receiver DeAndre Hopkins with 1:49 left in the first half, Smith led the Chiefs on a seven-play, 75-yard drive to extend the lead back to 16 points. The drive after Watson found Fuller for the long touchdown pass to help the Texans get within six points, Smith again led the Chiefs down the field for a touchdown.

Watson finished 16-of-31 for 261 yards and five touchdowns and no interceptions.

Watson is the first player in Texans history with four or more passing touchdowns in back-to-back games and, according to Elias Sports Bureau research, is just the second rookie in NFL history to do that, joining Robert Griffin III in 2012. He has multiple passing touchdowns for the third straight game, which is tied for the third-longest active streak in the NFL.

"I’m impressed," Hopkins said of Watson. "The thing that he can do with his feet and his arm as well. Everybody knows he can run, but still he looks downfield to make plays. The throws he made towards the end of the game were some special throws.”

Kansas City was 9-of-16 on third down. The Chiefs finished with 450 total yards of offense and 29 first downs.

The Texans’ offense may be able to help out more this season in Watt’s absence, but the defense needs to find the same success it found in 2016 replacing Watt’s production and presence on the field if Houston wants to continue to contend in the AFC South.

"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.

"That's the National Football League. Guys get hurt and the next guy in the team meeting room at that position needs to step up."