According to Adam Schefter of ESPN, the "team fears he tore his biceps," which would cost him the rest of the year.
Pittsburgh signed him to a five-year, $60 million extension Saturday, per Ian Rapoport of NFL Network.
Tuitt totaled one tackle for loss before coming out of the team's 21-18 win over the Cleveland Browns Sunday.
The lineman has been a consistent contributor on the Steelers defense, starting every game he has played over the past two seasons.
He has racked up 91 tackles and 10.5 sacks in this stretch, an impressive number for an interior player in a 3-4 system. This production comes despite missing a combined four games during the regular season.
The former second-round pick now wants to become even more valuable for Pittsburgh, putting effort into improving himself over worry about contract disputes.
"What I care about is staying in shape and being the best person I can be every day and enjoying what I'm doing," Tuitt said during the offseason, per Jeremy Fowler of ESPN.com. "... It's just preparing and becoming that guy, the guy the Steelers need me to be and help them get to the Super Bowl."
His high level of play only makes his absence more devastating when not on the field. The Steelers still have Cameron Heyward, but they will likely have to turn to Tyson Alualu and others for depth.