HOUSTON -- Although Bill O'Brien is pretty much set as head coach of the Houston Texans next season, there have been some noteworthy changes to his coaching staff.

The Texans have two coaches who have already been given permission to interview for jobs with other teams, and the team has also parted ways with others. Here's a look at where the coaching staff stands now:

Mike Vrabel: Vrabel was promoted from linebackers coach to defensive coordinator a year ago. That offseason, his name was in the mix for the Los Angeles Rams' head-coaching job. In January 2016, Vrabel reportedly turned down a job as the San Francisco 49ers' defensive coordinator to stay with the Texans, who promoted him to that position. The former linebacker interviewed with the Detroit Lions on Wednesday for their head-coaching job and could be a candidate for other open jobs, as well.

Although the Texans' defense took a step back this season, going from the No. 1-ranked unit in 2016 under Romeo Crennel to No. 20 in 2017, it was missing defensive end J.J. Watt and linebacker Whitney Mercilus and lost cornerback A.J. Bouye to free agency, among other departures to key contributors from '16. Vrabel does not have a ton of NFL coaching experience -- just three seasons, all with the Texans -- but he is well-liked and respected in Houston's locker room.

"We don't want to lose Mike, but these are things that happen every year," O'Brien said. "[When] you have a good staff, good core group of coaches, teams are going to come after them. Mike's a great leader, got good knowledge of the National Football League, good knowledge of our scheme. I wish him the best with the teams that are going to interview him, but we'd love to have him back here."

Sean Ryan: At the beginning of 2017, the Texans promoted Ryan from wide receivers coach to quarterbacks coach after parting ways with George Godsey. Teams have noticed the success Ryan had with rookie quarterback Deshaun Watson, which led to the Browns requesting permission to interview him for their opening at offensive coordinator.

"I feel like Sean has a real bright future in this league. He works very hard, does a lot of different things for me," O'Brien said. "I think Sean's goal is to call plays in the league, and that's why I don't personally want to stand in his way of having an opportunity to do that."

O'Brien calls the plays for Houston, and with Watson returning next season after a historic rookie campaign, that's unlikely to change in the near future.

"I think that's a great opportunity for him to be able to do that if the Browns take that permission and want to bring him in for an interview," O'Brien said. "I think at least that opportunity to interview is a good opportunity for him."

Romeo Crennel: The Texans moved Crennel from defensive coordinator to associate head coach in 2017. O’Brien said he "fully expect[s] [Crennel] to be back next year."

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The former Chiefs and Browns head man and longtime coach mentioned during the season that he enjoyed his new role, which allows him to not just work with the defense. But if Vrabel got a head-coaching job, there's a chance O'Brien would ask Crennel to move back to defensive coordinator.

Larry Izzo: The Texans parted ways with their special teams coordinator after the unit struggled in 2017.

"It was too inconsistent," O'Brien said of the Texans' special teams. "We have to look at it schematically, we have to look at it from a coaching standpoint. We've got to look at every facet of special teams because in order for us to get to where we get to, we have to improve that area. We've got to be better on special teams."

Assistant special teams coordinator Doug Colman will also not return. He and Izzo joined the Texans' staff in 2016 after the team fired Bob Ligashesky.

Charles London: Although O'Brien wanted London to stay on the staff, the running backs coach chose to leave to pursue other opportunities.

John Butler: A year after the Texans' secondary finished the season allowing the second-fewest passing yards per game, the unit went downhill in 2017. This season, the Texans ranked 24th, allowing an average of 237.4 passing yards per game. Part of the reason for that fall is because the team lost Bouye in free agency to the Jacksonville Jaguars and were not able to replace his production.