If his father indeed has serious health issues, I completely understand this. And I really hope that is the reason he is leaving (not hoping his father is ill obviously, just hoping Mike didn't have a bad experience here). Family always comes first and I would do the same if my father was ill.
Notwithstanding, I am going to be extremely critical of the staff here. Mitchell was healthy the second half of the season and was starting to blow up in practice. At that point, we were undefeated and on the mission for a NCG appearance. We were a couple defensive playmakers away at the LB and DB position away from being a defense that could make that happen and it was clear that Grant, Pitt Brown, etc. were not getting it done (see Bell and Mitchell). Mitchell was ready to play, should have played, and the same as Bell, could have taken his freshmen lumps while getting experience at the position and improve as the season went along. Given all of the above, there was zero reason to redshirt Mitchell. If he plays to half of his abilities, he won't be here for 4 years anyways and we needed him today, if not yesterday. If Mike is leaving in any part due to how his situation was handled (even if part of the reason is to be with his father), he is completely justified. In any event, my confidence in the LB play improving next year is substantially reduced now. With Shazier gone and now Mitchell, we are going to have to bank of the 2014 class to step it up big or hope that by some miracle something clicks with Grant and/or Perry.