Pittsburgh Steelers quarterback Ben Roethlisberger's shoulder injury is significant, and could force him to miss extended period, according to an ESPN.com report, citing a source close to the team. Roethlisberger went down hard on a sack by the Kansas City Chiefs' Justin Houston, on Pittsburgh's first possession of the third quarter, and didn't return. He had an MRI on his right shoulder at a hospital. A second on is scheduled for Tuesday. Early reports said Roethlisberger suffered a sprain, but a source told ESPN.com that doctors wanted further tests. A complete analysis won't be available until later Tuesday. The injury couldn't have come at a worse time. The Steelers face their chief rivals, the Baltimore Ravens, twice in the next three weeks. Byron Leftwich relieved Roethlisberger and went 7-of-14 for 73 yards. The Steelers beat Kansas City, 16-13, in overtime for their fourth straight win.