The injury would end his season. According to Rapoport, Keisel will have an MRI to confirm the tear.
Keisel left the team's game against the Saints in the third quarter.
"I'm really disappointed," veteran strong safety Troy Polamalu said. "I've seen it happen with Aaron (former Steelers defensive end Aaron Smith). There's very few guys here left in the locker room that you've experienced a lot with ups and downs, and you know that you can carry through with them. We're definitely going to miss (Keisel) and his leadership."
Free safety Mike Mitchell felt so bad for Keisel that he gave him a hug on the Steelers' sidelines after the third quarter.
"It almost makes you want to cry when you lose a player like that because he's so instrumental to who we are and what we try to do here," Mitchell said. "Everyone's got to pick it up."
In 11 games this season, Keisel has 14 combined tackles and a sack.
The Steelers re-signed Keisel in August. The 13-year veteran has played for the Steelers for his entire NFL career.
- Sarah Barshop