Ben Roethlisberger landed awkwardly when he was sacked in the third quarter. He left the game to go to the hospital for further evaluation.
The team has learned he has a sprained throwing shoulder, according to ESPN's Adam Schefter.
The Steelers have endured several injuries during the 2012 season but have responded and continue to win games.
But losing Roethlisberger would pose a more serious risk to Pittsburgh's playoff aspirations.
This isn't the first time that Roethlisberger has had injury problems this season. He's struggled with his throwing shoulder and had an MRI last week to check for damage. He was listed as day-to-day, as imaging didn't reveal any additional damage.
The Steelers offense has endured several injuries at other positions. Pittsburgh has rotated players at running back based on who is available. Rashard Mendenhall has played in just two games while Jonathan Dwyer and Isaac Redman have shared time carrying the load based on who is the healthiest.
The offense has continued to function with a revolving door at running back and changes along the offensive line. But it has worked because Roethlisberger is able to avoid pressure and deliver the ball down the field.
The defense has also dealt with injuries. Troy Polamalu has missed most of the season and the linebacker crew hasn't been healthy. The Steelers have enough depth on defense to remain competitive without their leader.
The same isn't true on offense.
Byron Leftwich replaced Roethlisberger, and the Steelers barely escaped with a win on Monday night. He helped move the team and engineered the drive that led to a Shaun Suisham field goal and a 13-10 lead.
But Leftwich is no Roethlisberger.
The throwing motion of Leftwich is slow and laborious. He doesn't recognize defensive looks at the same speed that Roethlisberger does, and he simply can't engineer points at the pace the offense needs. These issues will lead to turnovers against better defenses.
At some point a team will buckle under the burden of too many injuries. Pittsburgh has been admirable in how they've responded to adversity in 2012, but losing Roethlisberger could be the proverbial straw that takes down the camel.
Pittsburgh (6-3) will remain in contention for a wild-card berth, but if they are to challenge the Baltimore Ravens for the AFC North title they will need to get Roethlisberger back on the field.