It's will be the third time in the last four years Smith will have missed significant time due to injuries. Smith missed 11 games last season after he tore his rotator cuff in the team's fifth game. In 2007, he missed the better part of the year after tearing his right biceps muscle.
Pittsburgh's defense was dramatically worse in 2009 as it was in 2008, when Smith earned a trip to the Pro Bowl. In 2007, they lost three of their final five games, including a home playoff loss to Jacksonville.
The Steelers sit at the top of the AFC North with a 5-1 record, but without their best defensive lineman, fending off the charging Ravens will be especially tough.
Steelers defensive captain, LB James Farrior, called Smith's injury "devastating and heartbreaking" to the team, and there aren't many players in the league carrying more respect with him into the locker room than Smith.
The return of Brett Keisel will leave Smith's position likely filled by Ziggy Hood or Nick Eason. Who the Steelers may look to bring in for Smith's roster spot if he's placed on Injured Reserve is a matter of some conjecture.
NT/DE Steve McLendon was pulled from the practice squad to the active roster, replacing QB Dennis Dixon, earlier this season. He was then released and re-signed to the practice squad when QB Ben Roethlisberger returned to suspension. It seems as if he'd be the most likely to be placed on the active roster.
However, it wouldn't be surprising if PIttsburgh kicked Sonny Harris's tires again. He replaced Smith last year, and is currently on the Bills practice squad.
DE Travis Kirschke, a member of both of Pittsburgh's recent Super Bowl championship teams, is currently a free agent, and will definitely garner some interest from the team due to his familiarity with the defense.