If I'm counting right - ND returns 8 starters on Offense (Boykin, Williams, Mustipher). Having only to fill 3 spots should keep ND able to maintain on offense. Replacing Williams worries me - but I think the other two should be fine.
ND returns 6 starters on Defense (Tillery, Bonner, Tranquil, Coney, Love). This side might not be as easy. They have recruited DL's pretty well and have a lot in their 2 deep returning - I think they will be OK, at DB Todd Light does a great job and hopefully the return of a healthy Shaun Crawford, and an improving Dante Vaughn - they'll be OK. LB is a concern - I'm assuming its Bauer and Genmark, but even the best LB's struggle in their first full starting year.
If my numbers are correct - needing 3 on offense and 5 on defense, ND should be able to maintain. ND certainly seems to be maintaining recruiting well into the future for the next few years and developing players well.
Time will tell. I think the key will be RB, and LB - I think they are in good shape everywhere else.