From what I've read the greater concern that they have is the loss of two defensive tackles. The replacements were talked about as a serious step down in terms of power. That could be a huge problem for their run-stop ability.
Here's what some cursory research turned up:
AF runs a 3-4. Their starting NT, starting right DE, and one of their starting LBs are all out.
The starting NT was Ryan Gardner-- a very experienced 6'3'', 260 lb. senior. Replacing him is Ross Fleming-- a very inexperienced 6'3'' 250 lb. junior. He's played in eight games and accumulated 5 TTs (2 UT and 3 AT). Our interior OL outweigh his kid by 53 lbs (Cave), 61 lbs. (Robinson), and 60 lbs. (Watt).
The starting right DE was Zach Payne-- a very experienced 6'3'' 255 lb. senior. Replacing him is Daniel Vinson-- a 6'3'', 245 lb. sophomore with no first team experience. He played on the junior varsity (scout?) team last year as a freshman.
The starting LB was Patrick Hennessey-- a very experienced 6'2'', 255 lb. senior. Replacing him is Stephan Atrice-- a 6'1'',
200 lb. junior with
no experience at LB. He primarily played on ST and as a backup Safety over the last two years.
AF's defense is pretty banged up right now. They were undersized the begin with, especially for a 3-4, but the injuries have robbed them of serious size and experience in their front seven. ND should be able to run the ball straight down their throats.