Tuitt will need season-ending surgery, sources told ESPN's Chris Mortensen.
The team signed defensive end L.T. Walton to fill the open roster spot.
Losing Tuitt is a huge blow to a Steelers defense that's been dominant in the past couple of weeks. In the midst of his sixth season with the Steelers, Tuitt set the tone for the team's productive pass rush early. He recorded 3.5 sacks -- tied for second on the team -- in the first three weeks of the season, including 2.5 against the Seattle Seahawks.
He was on his way to outpacing his mark of 5.0 sacks in 2018.
"I think a lot of times it is him winning his battles," defensive coordinator Keith Butler said after the Seattle game. "Sometimes he gets around holes and gets upfield and just tears everything up.
"He's getting better and better every week for us."
After Tuitt's monster Week 3 performance, Butler said his defensive end was reaching the prime of his career after a couple of up-and-down seasons with the Steelers during which he sometimes tried to do too much.
"A lot of times it takes them two or three years to learn what's going on with a defense and where my help is and what I can do to take advantage of things and the more I can anticipate some offenses and what they are doing, the more I can anticipate, the faster I can play and the more plays I will make," Butler said. "And I think he is getting there."
In his last healthy game against Baltimore, Tuitt racked up eight tackles. He had 22 on the season.
Tuitt played just five snaps before suffering the injury, opening up more playing time for defensive end Tyson Alualu. The 10-year veteran figures to have an even more prominent role with Tuitt out for the season. He has seven tackles this season and a sack, recorded against the Cincinnati Bengals.