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.
Tomlin described Roethlisberger's shoulder injury as an "SC sprain." The exact terminology of Roethlisberger's injury is a sternoclavicular dislocation, according to Dr. Kaplan. In this type of injury, the central end of the clavicle separates from the sternum (chest bone). An uncommon injury, it is extremely painful with strong ligament tearing (sprain). The usual treatment for the injury is conservative (no surgery, with rest, sling and rehab). Typically it takes several weeks until a quarterback is able to return to passing accurately and with sufficient velocity.
Of the 137 games the Steelers have played since Roethlisberger's first season of 2004, he has missed 14, including his four-game suspension in 2010. In that span, Pittsburgh is 86-37 (.699) with its star QB and 9-5 (.643) without, according to ESPN Stats & Information.
The Steelers have managed to survive without Roethlisberger before. They are 7-5 in games without their franchise quarterback since 2005, including a 4-1 mark over the last two seasons. Roethlisberger missed the first four games of the 2010 season after being suspended for violating the league's personal conduct policy, but Pittsburgh started 3-1 behind Dennis Dixon and Charlie Batch, who will serve at Leftwich's backup on Sunday.
Batch filled in nicely last December when Roethlisberger was sidelined with an ankle injury, throwing for 208 yards in a 27-0 win over the St. Louis Rams.
Leftwich is 0-6 in his last six games as a starter, his last victory coming on Oct. 8, 2006 while playing the Jacksonville Jaguars, who selected Leftwich with the seventh overall pick of the 2003 draft.
When asked Tuesday how the playbook would change if Leftwich had to start, Tomlin said "I don't know that it does."
"I try to prepare as if I am the starter every week," Leftwich said. "Nothing will change. I wish Ben the best. I hope he is healthy. Other than that I will be ready to go."
Leftwich replaced Roethlisberger and finished 7-of-14 for 73 yards in Monday night's victory.
Sunday night's game will be for first place in the AFC North. The Steelers (6-3) have won four straight to pull within a game of first-place Baltimore (7-2).
Information from ESPN NFL Insiders Adam Schefter and Ed Werder, ESPN reporter Josina Anderson and The Associated Press was used in this report.