This is from the Pitts/Ravens game 2012...Line opened with Ben as the starter. Word came out he would miss game due to shoulder injury.
Pittsburgh went from a 3.5-point favorite to a 2.5-point underdog, indicating that The Greek, a well-respected sportsbook, sees the drop off from Big Ben to Leftwich as 6 points, in regards to setting the line.
Of course, every injury situation is different and specific line moves depend on a multitude of factors, like backup quarterback, opponent, shading for public overreaction, etc.