Coach Mike Tomlin revealed the injury at his Tuesday news briefing.
The injury is not expected to be career threatening, Tomlin
said. Smith was placed on injured reserve, a move that means he
cannot return this season. Defensive lineman Ryan McBean was
promoted to the 53-man roster.
"Being the guy he is, he continued on," Tomlin said Tuesday.
"He came out a few times, and that was worrisome because this guy
doesn't come out of games."
The Steelers are already playing without starting safeties Ryan Clark, who was placed on injured reserve earlier this season, and Troy Polamalu, who has missed the last three games. The additonal loss of Smith, one of the best two-way defensive ends in the league, is a significant blow to the NFL's top-rated defense.
Smith, 31, is a prototypical 3-4 end, a terrific anchor defender against the run who also has decent pass rush skills. On third down, the versatile Smith often moves inside to rush the passer.
Since 2000, when he moved into the lineup full-time, Smith had missed just four starts. He had missed two games earlier this season with a sprained ligament in his left knee.
It is uncertain who will replace Smith, a nine-year veteran, in the starting lineup. But the Steelers probably will be forced to use a combination of players -- likely Travis Kirschke, Nick Eason and Chris Hoke -- in Smith's absence.
In 11 games this season, Smith had 31 tackles and 2½ sacks.
For his career, the former Northern Colorado standout has recorded 421 tackles, 36½ sacks, seven forced fumbles, seven recoveries, one interception and 17 passes defensed. A fourth-round choice in the 1999 draft, Smith was named to the Pro Bowl in 2004, a season in which he registered eight sacks.
Len Pasquarelli is a senior writer for NFL.com. The Associated Press contributed to this report.