HOUSTON -- After finishing the season ranked 28th in the NFL in passing defense, the Houston Texans have some work to do on their secondary.

There is one place head coach Bill O'Brien would like general manager Brian Gaine to start.

"There's no doubt we would love to have Tyrann [Mathieu] back," O'Brien said last week at the NFL scouting combine.

And while the Texans head coach said several times that the free-agency aspect of the game is "a business" and that "it takes two to tango," there's no doubt Mathieu was one of few bright spots in Houston's pass defense last season.

The former Arizona Cardinals safety quickly became a leader in the locker room after joining the team last March and was a difference-maker on the field as well. Mathieu was brought in to play safety, but he was moved around the field because of injuries in the secondary. He bet on himself last offseason when he signed a one-year, $7 million contract, and it paid off. Mathieu is set to receive a big payday from someone this offseason.

"He's a guy that really meant a lot to our locker room," O'Brien said. "I've said it time and time again, this guy came in and was there from the offseason program and then at the end of training camp was voted a team captain. I think that says a lot about the impact he had, not just as a player -- he's a good football player, he's a smart football player, he's a versatile player -- but also, what he meant leadership-wise in the locker room."

The Texans have another veteran entering free agency in longtime Texans cornerback Kareem Jackson, who was the Texans' first-round pick in 2010. The veteran moved from corner to safety last offseason and had a lot of success at the position early in the season, but he ended up primarily playing cornerback, again, because of injuries.

"Kareem had a really good year for us, also," O'Brien said. "Again, he's a versatile player. He's a guy that has leadership qualities. He's been with us since the day I walked in here. He's an original Texan, was drafted as a Texan and has had a good career here."

Jackson is coming off of his best NFL season, with a career-high 87 tackles and 17 passes defended. He also had two interceptions, two forced fumbles, a fumble recovery and a sack. Though the team valued what Jackson did last season, it seems unlikely it would bring back both Mathieu and Jackson, especially if Jackson returns to playing safety. With Justin Reid coming off a successful rookie season, only one of Jackson or Mathieu is necessary.

The Texans' first noteworthy roster move of the offseason was in their secondary. On Tuesday they released Kevin Johnson, who would have been owed more than $9 million for his fifth-year option if he was on the roster at the beginning of the new NFL year. The 2015 first-round pick missed the majority of the 2018 season after suffering a concussion in the opener against the Patriots and has played in only 35 games in his first four seasons. While it's possible Johnson could return on a cheaper contract, the Texans were not willing to pay him top dollar at the position; he was set to make more per season than veteran cornerbacks Johnathan Joseph and Aaron Colvin.

Joseph will return for the second year of the two-year, $10 million contract he signed a year ago after a strong season at age 34. Houston will hope for more from Colvin, who signed a four-year deal worth $34 million in 2018. Colvin missed time last season after injuring his ankle, but even after he returned, he did not play at the level the Texans were hoping for when they signed him and was a healthy scratch down the stretch.

"[Colvin] probably expected himself to play and perform better, too," Gaine said. "I would say this: Aaron dealt with some injury last year. Unfortunately, he tried to come back from that injury and probably didn't play as well as he hoped, so we're hoping Year 2 of free agency, you get a better dividend and more contributions from the nickel position."

The front seven is obviously the highlight and focal point of the Houston defense right now -- it's hard for that not to be the case when J.J. Watt and Jadeveon Clowney are lining up together -- but it's not hard to see just how good the unit could be. The defense gave up an average of just 19.8 points per game last season, tied for fourth in the NFL, despite all the changes in the secondary. In 2016, Houston thrived defensively with a cornerback trio of Jackson, Joseph and A.J. Bouye. Since Bouye's departure, there has been a big hole in the secondary.

"On the defensive side of the ball, the strength of our team, obviously, sits in the front seven. As much as you can pressure the quarterback, stress the quarterback, stress the football, that's critically important," Gaine said. "At corner, we're dealing with some free-agent situations and we have to navigate that over the next coming weeks. So, we'll continue to try to build the defense and make it a strength of the team."