PITTSBURGH -- Playing for a month without Troy Polamalu was difficult enough for the Steelers. Playing the rest of the season without Aaron Smith might be an equally big challenge for one of the NFL's best defenses.
On the same day that Polamalu returned to practice after missing four games with a knee injury, the Steelers scrambled Wednesday to replace Smith, the run-smothering defensive end who was placed on season-ending injured reserve with a torn rotator cuff in his right shoulder.
 For more on the Pittsburgh Steelers, check out the latest from our bloggers.
Smith has been to the Pro Bowl only once in 11 seasons and isn't as well-known nationally as Polamalu, the five-time Pro Bowl safety, or linebacker James Harrison, the 2008 NFL Defensive Player of the Year. But several Steelers said Smith might be the most difficult defensive player to replace because he's so critical not only to stopping the run but to keeping the line of scrimmage under control for the pass rush to work.
"A guy like that, I truly believe, is irreplaceable," Polamalu said. "There's a lot of other big role positions that get a lot of prestige, but who can always be replaced -- myself included -- but it's always tough when it has to do with our line. I truly believe our defensive line controls the outcome of the game. It's big. It hurts us."
The Steelers found out late in the 2007 season what can happen when Smith is out. They were No. 2 against the run when Smith tore a biceps muscle during a game against the New England Patriots, then gave up 629 rushing yards while losing three of their final four games. The Jacksonville Jaguars beat the Steelers twice in Pittsburgh -- once in December and again in a wild-card playoff game three weeks later.
Steelers linebacker James Farrior, who usually lines up behind Smith, said it's obvious what losing Smith means to a defense that is ranked fourth overall in the NFL and second against the run.
"You don't have that dominant player that we had," Farrior said. "It's like when Troy was out, we didn't have that big playmaker in the secondary, and you see that sometimes during games."
After Polamalu tore the medial collateral ligament in his left knee during the Sept. 10 season opener against the Tennessee Titans, the Steelers lost their next two games in the closing seconds to the Chicago Bears and Cincinnati Bengals, gave up 21 points in the fourth quarter while beating the San Diego Chargers and scrambled to put away the Detroit Lions.
Smith rarely makes the high-impact plays that Polamalu does, but safety Ryan Clark said the Steelers wouldn't have been No. 1 in the league the last two seasons without him. Last season, the Steelers narrowly missed becoming the first team since the 1970 NFL merger to lead the league in fewest points, total yards, rushing yards and passing yards allowed.
"Nobody's going to replace him," Clark said. "I think he's one of the unsung heroes in this league, a guy that's very underappreciated for what he's done for many years."
This season, Smith has 13 tackles, two sacks and a blocked field goal. He is eighth in team history with 43 sacks.
To replace the 6-foot-5, 298-pound Smith on their 53-man roster, the Steelers re-signed Ra'Shon "Sunny" Harris, a sixth-round draft pick they cut in September. Harris had been on the Carolina Panthers' practice squad.
First-round draft pick Ziggy Hood and longtime backup Travis Kirschke probably will receive most of the playing time for Smith, starting Sunday against the Cleveland Browns.
Steelers coach Mike Tomlin said Hood isn't advanced enough to take over the job full time, and the rookie agreed.
"For right now, I'm just holding a spot until he (Smith) gets healthy," Hood said. "I can only do so much right now, there's so much I've got to learn, a lot more I need to know about the defense. But everything I've learned -- from my stance, shedding blocks -- I learned from him. I watch him and try to study him as much as I can and try to mimic him."
Farrior said there's one way for the Steelers to react to Smith's loss.
"We need to win," he said. "We need to win so guys won't talk about Aaron being out."
Copyright 2009 by The Associated Press