Byron Leftwich will start for the Pittsburgh Steelers at quarterback against the Baltimore Ravens on Sunday night as Ben Roethlisberger sits out due to injuries, the team announced Wednesday.

Roethlisberger, who has been diagnosed with a sprained shoulder, also suffered damage to his ribs Monday night, which a source familiar with the injury described to ESPN as rare and dangerous and of greater concern to the Steelers and team doctors than his shoulder injury.

Doctors are concerned that the quarterback's rib injury could lead to more internal damage if he were to play anytime soon.

According to ESPN medical analyst Dr. Michael Kaplan, Roethlisberger's fractured rib has the potential to tear lung lining (pneumothorax), similar to an injury sustained by the Cowboys' Tony Romo last year. The broken rib edge is sharp and if a second hit were to displace it inward, internal damage is possible. When Roethlisberger's shoulder and rib heal sufficiently, he likely will use a flak jacket for protection upon his return to the field.

Roethlisberger suffered the rib injury on the same hit on which his shoulder was injured.

The Steelers' decision to be cautious with their franchise quarterback doesn't come as a surprise. A Steelers source had told ESPN on Monday that the team's staff was told: "Proceed with (backup quarterback) Byron (Leftwich). It doesn't look like you're going to have Ben this week. We're looking at next week as a possibility as of right now."

At his news conference Tuesday, coach Mike Tomlin called Pittsburgh's franchise quarterback "questionable" but otherwise offered little detail, less than 24 hours after Roethlisberger was pounded into the ground by Chiefs linebackers Tamba Hali and Justin Houston in the third quarter of Pittsburgh's 16-13 overtime victory.