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

The Steelers (5-2) already were without defensive end Aaron Smith, a premier run-stopper who's sidelined 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 keep him out another couple of weeks. He was injured during a 27-17 victory over the Minnesota Vikings 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 Pittsburgh 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 he's expected to receive 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 has allowed the fewest rushing yards in the NFL this season.

"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 the Carolina Panthers' 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 receive more playing time than usual in the middle as Hampton takes off some plays.

"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 believe 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," Steelers 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 has 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 have allowed 76.6 rushing yards this season, the Broncos are averaging 123.1, 11th best in the league. Rookie Knowshon Moreno has run for 420 yards, and veteran Correll Buckhalter has gained 329 yards and averaged 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."