I got the impression from today's comments on Stanley is that he's in a Flacco-like situation where the injury doesn't appear to be long-term but putting him in risk would be a huge problem.
The way I see it is that there are really only 3 players who, if significant injury hit, would basically foreclose any success this year: Flacco, Stanley, Mosley. Losing other guys definitely hurts, but losing one of those three probably drops what could otherwise be a 9-7 season to a 5-11 season or worse- primarily because of the lack of depth behind each of them and the importance of their positions. The Ravens also probably view certain players as indispensable- its probably a longer list than mine, but I would guarantee that Stanley is a guy they aren't going to risk even a little before the regular season.