If he does not return, then the only 5th year guys in that class (not counting walkons and transfers) will be Sneed and Craig.
And there will be only 14 forth-year Seniors, provided no one else enters the Portal. And you can be sure that some will either this year or next.
I cannot remember there ever being that few. So much for building a team primarily by recruiting and developing high school recruits. Whoever says ND does not need to rely much on bringing in transfers is either ignorant or dishonest.