ASSOCIATED PRESS Carolina Panthers tight end Dante Rosario (88) catches a pass behind Pittsburgh Steelers safety Troy Polamalu (43) for a first down in the first quarter during a preseason game in Pittsburgh on Thursday night. The Steelers won, 19-3.

PITTSBURGH (AP) — All of a sudden, losing Ben Roethlisberger for at least a month isn’t the Steelers’ only quarterback worry. Byron Leftwich, expected to be the fill-in starter, is injured and looks to be out indefinitely.

Leftwich, tuning up for his anticipated Sept. 12 start against Atlanta, sprained his left knee in the second quarter as Pittsburgh beat the starter-less Carolina Panthers 19-3 on Thursday night.

Coach Mike Tomlin said Leftwich would undergo an MRI on Friday, but the quarterback’s teammates sounded discouraged. Backup Charlie Batch called it a ‘‘serious’’ injury, and Dennis Dixon — now the likely starter — said he couldn’t talk to Leftwich because doctors were working on the knee so intensely.

‘‘It’s an opportunity, I guess, but it’s unfortunate for Byron for what he went through. But this team has to continue to go forward,’’ said Dixon, who has started only one NFL gam.