PITTSBURGH (AP) -The thin air in Denver isn't as much of a concern to the Pittsburgh Steelers as their rapidly thinning defensive line.

The Steelers (5-2) already were without defensive end Aaron Smith, a premier run-stopper who is out for the rest of the season with a torn right rotator cuff. Now they're missing his replacement, Travis Kirschke, who has a torn left calf muscle that could sideline him another couple of weeks. He was injured during a 27-17 victory over Minnesota on Oct. 25.

One of the strengths of a Steelers defense that was the NFL's best the last two seasons is its depth, but that's all but vanished now that Smith and Kirschke are hurt. Now, the Steelers must depend on two rookies and a player who was cut last month.

Welcome to the NFL, Ziggy Hood and Sunny Harris. Welcome back, Nick Eason.

Hood, a first-round draft pick, won't start against the Broncos (6-1) on Monday night but is expected to get significant playing time because teams traditionally rotate linemen in Denver's high altitude. Eason, a former Broncos draft pick, will move from right defensive end, where he backs up Brett Keisel, to start at left defensive end. Harris is the other backup.

Only a month ago, Eason was let go so the Steelers could activate an extra running back. Suddenly, he is starting on the defense that allows the fewest rushing yards in the NFL.

``I was released the fourth game and the next thing you know I'm playing,'' Eason said Wednesday. ``Now I'll be playing a lot more. My snaps have doubled in three weeks. It's amazing, but that's the life story in the NFL.''

Harris, a sixth-round draft pick, has a similar story. He was released after training camp only to re-sign two weeks ago after spending time on Carolina's practice squad.

Because of the injuries, the Steelers must go into one of their biggest games of the season with only one experienced backup defensive lineman in nose tackle Chris Hoke, who plays behind Casey Hampton. Hoke can play defensive end, but it's likely he'll get more playing time than usual in the middle as Hampton takes some plays off.

``We go through this every year,'' Keisel said. ``Last year I was out for a few games and Aaron missed some games (in 2007). We have a lot of guys who know how to play.''

Or at least the Steelers think they know how to play.

Hood, a former star at Missouri, has taken a minimal number of snaps this season. He usually spends his practice weeks on the scout team that replicates the opposing defense.

``He's a tough guys to block on Wednesdays and Thursdays,'' coach Mike Tomlin said. ``At some point, he's going to be a tough guy to block on Sundays. We'll see when that happens.''

Hood said it's been a lot to absorb as a rookie, keeping up on the Steelers' ever-changing defense while also learning a different style each week during practice.

``Each day, I see I'm somewhat getting better, not only mentally but physically as well,'' Hood said. ``I believe I've taken the necessary steps to get better. When the starters need to take a breather, I'll be able to fill in until they get better and go 100 percent every snap.''

While the Steelers allow an average of 76.6 yards rushing, the Broncos are averaging 123.1, 11th best in the league. Rookie Knowshon Moreno has run for 420 yards and former Eagles running back Correll Buckhalter has gained 329 yards and averages 6 yards per carry.

``I've told a lot of young guys, 'The only way you can get prepared to play is to play,''' Eason said. ``You can practice all week, but when you get in a game it's a different mentality.

``It's a lot more pressure, there's the fans, it's different - it's 100 miles per hour and it's full-go tackling. And that's the only way that guys like Ziggy and Sunny can get playing experience.''Copyright © 2005 The Associated Press. All rights reserved. The information contained in the AP News report may not be published, broadcast, rewritten or redistributed without the prior written authority of The Associated Press.