Didn't Big Ben have the same type of rib injury last year late in the middle part of the season? I am almost sure of it, I had him as a starting QB on one of my fantasy teams. Since it is close to the heart, and it being broken could KILL the man, they kept him out for like 4 games.
If it is the same type of injury, we might see Cassel again next game for safety issues alone; we all might not like Ponder's game time skills, but I do not think anyone wants him to bleed out due to a big hit.
It was similar in the location, but Big Ben's was a dislocated rib.