"You've got to say Charlie's in pole position because he's got a clean bill of health," Tomlin said.
Dixon tore cartilage in his left knee in Sunday's 19-11 win at Tennessee. He had been filling for regular quarterback Ben Roethlisberger, who is serving a four-game NFL suspension.
"We're going to wait and see what it looks like (after the surgery) before we proceed with the length of time he can potentially be out," Tomlin said of Dixon.
The Steelers re-signed Leftwich after releasing him on Saturday. He played with Tampa Bay in 2009 after being a back-up with the Steelers in 2008.
Leftwich's left knee, which he sprained earlier this month, should not be a factor in whether he plays, Tomlin said.
"He's been able to comfortably get through some workouts," the coach said.
(Reporting by Gene Cherry in Raleigh, North Carolina; Editing by Steve Ginsburg)