PITTSBURGH — Right arm tucked into a black sling, weary eyes betraying a decided lack of sleep, Ben Roethlisberger tried to stay positive after the worst — and by far the most harrowing — injury of his career.

The Pittsburgh Steelers quarterback believes he can return this season despite spraining his right shoulder and suffering what he called a dislocated rib in Monday night’s overtime win against Kansas City.

He’s just not sure when he’ll be ready. And his medical team appears nowhere close to figuring out how exactly to get him there.

“From what (the doctor) said he’s trying to talk to experts because there is no case study over the exact same thing,” Roethlisberger said Wednesday. “We’re just trying to talk to people ... because we don’t know.”

All Roethlisberger knows for certain is that doctors are concerned the rib — which he injured while getting sacked by a pair of Kansas City linebackers in the third quarter on Monday — could puncture his aorta if jostled around.

He also knows he’ll be a spectator indefinitely while the surging Steelers (6-3) try to track down AFC North-leading Baltimore (7-2). The rivals meet twice over the next three weeks, with the first showdown coming Sunday night at Heinz Field. Veteran backup Byron Leftwich will take the snaps in Roethlisberger’s place, looking for his first win as a starter in six years.

“I’m not going to go out there and try and be Ben,” Leftwich said. “We see the game differently. He’s physically able to do some things that I can’t do but that doesn’t mean I can’t go out there and do my job.”

The Steelers have been forced to play with Roethlisberger occasionally over the last nine seasons, going 8-5 without their franchise cornerstone since 2004. Four of those losses, however, have come against the Ravens.

Still, Roethlisberger remains optimistic he’ll be back to work at some point over the next seven weeks. Asked if he thinks the injury is season-ending, the two-time Super Bowl winner shrugged his one good shoulder and attempted to remain upbeat.

“I don’t think so, I don’t know though but I’m not a medical expert,” Roethlisberger said. “I just know I’m going to do what I can to get back.”

At the moment, doing nothing appears to be the smartest course after his season came to a stunning halt early in the third quarter against the Chiefs.

Roethlisberger was stepping up in the pocket trying to extend a third-down play when Kansas City linebacker Justin Houston wrapped up his legs and teammate Tamba Hali drove all of his 265 pounds into Roethlisberger’s chest and slammed him to the turf. The quarterback’s right arm dug into his side at the moment of impact, dislocating the rib and sending a jagged edge perilously close to the aorta.

“When I hit the ground is kind of when I felt something not right, like a crunch or a crack,” Roethlisberger said. “It’s kind of hard to explain.”

What’s easy to explain is the pain, which Roethlisberger described as “nine on a scale of 1-10.” Most of the misery is coming from the rib and he joked he’ll likely have to hold his son — who is due to arrive sometime in the next few weeks — with in his left arm instead of his right.

Unless Roethlisberger learns to throw with his left arm too, Pittsburgh’s playoff chances will rest with Leftwich. The former first-round pick hasn’t started a game in three years and hasn’t won one since 2006. He went 7 of 14 for 73 yards in relief against the Chiefs, leading Pittsburgh to a go-ahead score in the fourth quarter.

“My first few plays I felt like everybody was moving faster than me,” Leftwich said. “Maybe that is because I’m slower than everybody. When you ain’t out there, let’s just be honest, it just takes awhile to get back.”

A full week of practice and the fact he’s spent four of the last five seasons as Roethlisberger’s primary backup means Leftwich isn’t exactly starting from scratch.

Early in his career, Leftwich was one of the most promising quarterbacks in the league. Taken with the seventh overall pick by Jacksonville in the 2003 draft, Leftwich led the Jaguars to a playoff berth in 2005 before things fell apart. He lost his starting job to David Garrard a year later then bounced around between Atlanta, Pittsburgh and Tampa Bay before returning to the Steelers for good in 2010.

Leftwich missed all but one game in 2010 due to a knee injury and spent all of 2011 on injured reserve after breaking his arm during a preseason game. Yet the Steelers have kept him around because of his close relationship with Roethlisberger and his arm, which remains one of the strongest in the league. Leftwich actually overthrew speedy wide receiver Mike Wallace on a fly pattern against the Chiefs, a rare feat.

“We have all the confidence in the world in Byron,” Wallace said. “He’s been in this league a long time, and he’s played a lot of games. We just need to work on some things in practice this week with him at quarterback, and I’m sure we’ll be ready to go by the time the game comes around.”

Roethlisberger will be there in Leftwich’s ear offering assistance when necessary, just as Leftwich has done for him through the years.

“He can make every throw on the football field,” Roethlisberger said. “He’s going to do just as good if not better.”

NOTES: DE Brett Keisel (shoulder), LT Max Starks (ankle), S Troy Polamalu (calf), RT Marcus Gilbert (ankle), WR Antonio Brown (ankle) LB Chris Carter (abdomen) and Roethlisberger did not practice on Wednesday. ... S Ryan Clark was limited in practice after sustaining a concussion against the Chiefs. Clark will meet with team doctors on Thursday for further evaluation.