Dixon, who started one game last season, got the assignment after Roethlisberger received a four-game suspension due to sexual misconduct allegations and Byron Leftwich injured his knee.
"Dennis has had a very productive preseason and training camp," Tomlin told reporters Monday. "He wedged himself into a battle and got some first team reps in some preseason games. Largely, we've been very impressed with how he's handling himself and the situation."
Leftwich was pegged as the starter for Sunday's season opener against Atlanta but sprained a knee ligament in the Steelers' last preseason game.