HOUSTON — The Houston Texans proved they were serious about upgrading their offense this offseason by luring Brock Osweiler from Denver with a four-year, $72 million contract. Now it’s time to see if the quarterback was worth it.

All eyes will be on Osweiler when the Texans begin training camp on July 31 — especially with star defensive end J.J. Watt sidelined by a back injury.

“I’m impressed with Brock, probably most of all in his ability to learn our system, our operation,” coach Bill O’Brien said. “It’s not easy to come in here and learn how we operate as an offense, how we meet, how we call plays at the line of scrimmage, how we teach defense … we’re really happy with his progress.”

On Thursday night, a person familiar with Watt’s condition said the star defender has had back surgery and will begin training camp on the physically unable to perform list. The person spoke to The Associated Press on on condition of anonymity because the team hasn’t confirmed the surgery or Watt’s roster status.

Watt, the NFL sacks leader last season with 17 1/2, will miss the start of training camp, but is expected to be ready for the season opener. Watt spent most of the offseason recovering after having surgery to repair a groin injury in January.

Watt’s still looking to do more in 2016 and said he never watches his highlights, instead viewing his bad plays to figure out how to get better.

“There’s so many great players in this league, and you want to learn and grow as much as you can and gain as much information as you can so that you can improve your own game,” Watt said.

Osweiler has started just seven games in his four-year NFL career and they were all last season. He was presumed to be Denver’s starter this season after Peyton Manning’s retirement, but he signed with Houston for megabucks instead.

He joins an offense highlighted by star receiver DeAndre Hopkins, who finished third in the NFL with 1,521 yards receiving last season, and that added running back Lamar Miller in free agency, then got receivers Will Fuller and Braxton Miller in the draft.

The Texans overcame quarterback woes last season to finish 9-7 and reach the playoffs for the first time since 2012. They were ousted in the wild-card round by Kansas City after Brian Hoyer committed five turnovers in the 30-0 loss.

Now Osweiler takes over and he feels good about his grasp of Houston’s offense after the work he got in practices and minicamp this spring and early this summer. He’s looking to take his understanding to another level in camp.

“It’s just focusing on the details of our offense,” he said. “There’s a lot of things that go into it — formations, motions, shifts, different cadences, different route concepts, run checks, pass checks — and so it’s just focusing on the details and focusing on the line-of-scrimmage operation.”

Some things to know about the Texans, who open the season Sept. 11 against the Chicago Bears:

EXPECTATIONS FOR CLOWNEY: Outside linebacker Jadeveon Clowney is entering his third NFL season and so far the No. 1 overall pick in the 2014 draft has failed to live up to expectations during two injury-plagued seasons. Clowney played just four games as a rookie before having season-ending microfracture surgery. He recovered from that for the start of last season, but missed three games and Houston’s playoff match with various ailments. He did show flashes of the potential he displayed in college, though, and finished with 4 1/2 sacks and forced a fumble.

“It’s frustrating. It feels like you’re letting the team down when you’re not out there,” Clowney said. “Injuries happen, you’ve just got to be mentally tough to come back and show the guys you are ready to play.”

MILLER’S POTENTIAL: The Texans snagged an intriguing player when they drafted Braxton Miller in the third round. Miller was the two-time Big Ten offensive player of the year in 2012 and 2013 after throwing from more than 2,000 yards and rushing for more than 1,000 in both of those seasons. He sat out for a season with a shoulder injury before moving to receiver last year, when he finished with 26 receptions for 341 yards.

Miller is happy with his progress since making the move.

NEW CENTER: The Texans will likely have rookie second-round pick Nick Martin at center after Ben Jones, who started every game the last two seasons, signed with Titans. He is the younger brother of Zack Martin, a first-round pick by the Cowboys in 2014 and a two-time Pro-Bowler at guard.