by John McClain, Houston Chronicle , The Houston Chronicle

We pretty much know what will be expected from an offense that ranked third in yards and 17th in points - reduced turnovers and improvement in the red zone.

But the defense is another matter with Bush's promotion, along with the hiring of Bill Kollar as assistant head coach/defensive line coach and David Gibbs as the secondary coach.

Bush was promoted and Kollar was hired in correlation with an anemic pass rush that produced only 25 sacks - 12 by Mario Williams. The Texans ranked 27th in sacks, and it's up to Bush and Kollar to improve pressure on the quarterback.

Changes are coming to a defense that ranked 22nd in yards (336.6) and 27th in points (24.6). Those tweaks will be in the line and at linebacker because Bush knows that improvement in the front seven, specifically the pass rush, means a more efficient secondary.

With the addition of four free agents and five draft choices on defense, expect differences in the roster and the starting lineup.

Sizing them up

Outside linebacker Brian Cushing, their first-round draft choice, will start on the strong side. And expect Zac Diles to compete for the starting job on the weak side when he recovers from his season-ending broken leg.

Drafted in the seventh round in 2007 to back up DeMeco Ryans at middle linebacker, Diles played so well the coaches moved him to the strong side in his second season. At the time of the injury, he was leading the team in tackles. He should be 100 percent by training camp, which begins July 31.

The idea is to get the best players on the field. Diles has proven he can play in the middle and on the strong side. Because he's quick, aggressive and makes plays from sideline to sideline, he often plays like an outside linebacker on the weak side.

If Diles is able to beat out Xavier Adibi for the other starting spot, he'll give the Texans more size, toughness and experience.

Ryans (6-1, 250), Diles (6-0, 240) and Cushing (6-3, 243) have the kind of size the coaches like. Adibi, Kevin Bentley and Cato June would provide depth.

As far as the defensive line, left to right, the starters should be Antonio Smith, Travis Johnson, Amobi Okoye and Mario Williams.

In passing situations, expect Smith to move inside and replace Johnson. Smith would be replaced by Tim Bulman, who finished second in sacks last season with four.

Barwin could be steal

The coaches want the four best rushers on the field in passing situations, and, eventually, they could be - left to right - Williams, Smith, Bulman and Connor Barwin, the second-round pick from Cincinnati.

Barwin, who played end only one season at Cincinnati, will stay on the right side. He's going to be behind when he's able to report, since he's going to miss OTAs because of Cincinnati's late graduation date. He should be available for minicamp on June 15-17.

Barwin (6-4, 256) has outstanding potential as a pass rusher. Once he proves it, he'll be on the field. That means Williams would move to the left side. Before Barwin was available in the second round, the coaches wanted to keep Williams on the right side if that says anything.

The Texans had a first-round grade on Barwin, and they think he has a chance to be special as a pass rusher. However, they know they'll have to be patient because of his lack of experience on defense.

One of the best story lines of training camp will be Kollar trying to get Barwin ready to contribute in passing situations.

John McClain can be heard on 610 AM at 7:30 a.m. Mondays and at 4:30 p.m. Fridays. john.mcclain@chron.com

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