HOUSTON -- Romeo Crennel didn't have to be convinced to return to his previous role as defensive coordinator and have the chance to coach the Houston Texans' defense back to one of the league's best.

It was his defense, after all.

"Honestly it's always been RAC [Romeo Crennel]'s defense, his philosophy and all that," outside linebacker Whitney Mercilus said. "I think everybody always questions as far as, OK, is it [Mike] Vrabel's defense to Romeo's defense and all that ... I just don't understand that. RAC always had the defense as far as his mentality on the defensive side, as well as the scheme, the plays that we play and things like that."

In January 2017, the Texans promoted Crennel to assistant head coach and moved Vrabel from linebackers coach to defensive coordinator. The unit finished No. 1 in total defense in 2016 under Crennel, but fell to 20th under Vrabel, although Vrabel's unit had several injuries to key players and lost shutdown corner A.J. Bouye in free agency. Despite the struggles Houston's defense had in 2017, the Tennessee Titans hired Vrabel as their head coach following the season.

With a vacancy at defensive coordinator, coach Bill O'Brien once again turned to Crennel.

In his role as assistant head coach, Crennel had a lot of responsibility -- and O'Brien certainly leaned on him -- but Crennel didn't have the pressure of calling plays for the defense.

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"I was less stressed last year," Crennel said. "There's stress involved in calling the plays and preparing weekly for the game plan. I sat in and listened and all that good stuff, but when you know you don't have that pressure, it's a little bit easier."

The shuffling at defensive coordinator brought tweaks to the scheme, but the unit's foundation has not changed.

Crennel, 70, has a different football style than Vrabel -- "a little more old-school," cornerback Johnathan Joseph said. Mercilus and Joseph described Crennel as a father figure, his calm but passionate demeanor a strength in his coaching.

"He has such a kind heart and he knows how to talk to each guy and to get through to each guy, but he also knows how to get on your ass if he has to and how to light a fire under you," J.J. Watt said. "He knows how to put us in good situations, he knows how to put us in situations that will be successful for us. I love the guy.

"Every single morning when he walks in, I'm in there, and he comes and taps me on the leg and he says 'How you doing today' and we have a little five-minute chat, and every single day it's some cool little five-minute chat that I get to have with RAC."

Defensive line coach Anthony Weaver called Crennel "a very calming influence" on the field.

"For all the knowledge he has in terms of schematics and X's and O's, I think more the demeanor that he brings -- the players feed off that," Weaver said. "It's a quiet confidence, and I think you see our defense go out and play exactly the same way."

That mindset and Crennel's presence was meaningful for safety Tyrann Mathieu when he was looking for a team after he was cut by the Arizona Cardinals in March. He said his meeting with Crennel, when the coach talked about his plans for Mathieu and making sure he found a good fit in Houston, was important.

"I don't think I've ever had a coach with his kind of pedigree, with all of the Super Bowl wins he's had and all the games he's coached in and being a head coach and being a defensive coordinator," Mathieu said. "Just talking to him, I think he has some great ideas for me. I think we're both going to feed off of each other, and at the end of the day, I want him to make me the best safety in the game, and I do believe I can get to that point."

The players who have been around Crennel since he joined O'Brien's staff in 2014 have seen how Crennel's belief in them has made a huge difference in their lives and with the team.

"Guys have that much respect to where you don't want to do the wrong thing, because you know how much time and the passion he has for the game," Joseph said. "You want to give your best effort every time out there for Romeo."