The team believes veteran defensive end Brett Keisel has torn his triceps, NFL Media Insider Ian Rapoport reports, via two sources informed of the injury.
Keisel will undergo an MRI to confirm that his season is over.
Unsigned through training camp, Keisel quickly earned his starting job and played well enough to hold off a promising Stephon Tuitt until Sunday.
The Steelers haven't been confident in Tuitt's run defense, so the second-round rookie might be used in a rotation with Cam Thomas going forward.
This could be the end of the line for Keisel, a key member of Dick LeBeau's defenses which ranked first or second in yards six times over the past decade. The hirsute wonder turned 36 years old during the season and is unsigned for 2015.