Keisel left the game in the third quarter, and coach Mike Tomlin said after the Steelers' latest loss to a team with a losing record that the 13th-year veteran will be further evaluated.
But teammates spoke of Keisel in the past tense after the Steelers fell to 7-5.
"You don't want to see anyone get hurt, especially someone like that where he won't be back for the rest of the season," outside linebacker James Harrison said.
Keisel re-signed with the Steelers in August -- his two-year, $3 million contract included a $500,000 signing bonus -- and he has been a relative bright spot on a defense that has struggled this season. Keisel entered Sunday's game with 12 quarterback hurries, a sack and an interception. He nearly sacked Saints quarterback Drew Brees early in the game and recorded three tackles before getting hurt.
"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."
The loss of Keisel delivers a blow to a defense that is just starting to get healthy, but the Steelers avoided a serious injury on the other side of the ball.
Quarterback Ben Roethlisberger hurt his throwing hand in the first quarter, but the 11th-year veteran stayed in the game.
He handed off to running back Le'Veon Bell several times with his left hand and at the end of the first quarter, Dr. James Bradley and head trainer John Norwig talked to Roethlisberger on the Steelers' sidelines.
Roethlisberger, however, walked away from both after a brief chat, and he didn't wear anything on his right hand after the injury. He also dismissed questions about whether hurting his hand affected his throwing after a game in which a handful of balls sailed on Roethlisberger.
Roethlisberger completed just 55.2 percent of his passes and tossed a pair of interceptions.
"Does it hurt? Yeah?" Roethlisberger said. "Did it affect my throwing? No."