Harrison has been sidelined during the team's 1-2 start with discomfort in his knee following surgery in August. The All-Pro linebacker returned to practice last Tuesday but left the team's facility Wednesday because of what his agent, Bill Parise, described as "a little bit of a setback" with the knee.
Pittsburgh's defense is ranked sixth in the league, but it hasn't appeared that way on the field.
The Steelers rank 30th in the league with five sacks and Harrison's replacements -- a combination of Chris Carter and Jason Worilds -- have combined for just one. Harrison's ability to heat-seek the quarterback is missed, but probably not by Michael Vick, whose Philadelphia Eagles wouldn't mind the defender's absence to drag on another week.