It had to happen eventually. They've been blessed by not getting too many injuries throughout the year (Winfield and Henderson), now it's hitting them at the worst possible moment.
I don't think that it'll be that big of a problem when Edwards is out. If you ask me, he's been benefitting from the play on the rest of the D-line. The Matt Cassel of the DEs, if you will.
Kevin Williams, on the other hand, would be a huge hit. For a DT, especially with the weigth he has, he's an amazing pass rusher. Pat is a good DT, but doesn't do a whole lot more than stumbling into the QB once or twice per year.
Percy Harvin would be quite a blow, as well. Both on offense and special teams.
Eventually, I think that all 3 of them will play. Harvin has had migraines all year long and has played in most of the games. Williams is practicing and Edwards will be back.
Although it would be interesting to see Allen - Kennedy - P. Williams - Robison on the field.