By ALAN ROBINSON
AP Sports Writer

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."