I'm gonna go with Evenrude on this one. This is the first loss I really was left doubting the coaches afterward. If Nebraska doesn't win one of the next two and misses going to a bowl again there will be a lot of pressure put on to fire Bill, but I can guarantee you he will get at least another year, which I think is fair.
Solich, you might remember, wasn't fired for not winning 10+ games every year. He was fired because we got blown out in all three losses. Texas, Missouri, and K-State (at home) all won by three touchdowns or more. I don't think firing him was wrong, nor was hiring someone with an offensive philosophy that was a lot different. People forget Nebraska wasn't strictly a power option team until the 90's.
Anyway, Callahan has shown he can recruit. I have yet to see proof he can coach well. Hopefully the next couple games and next season will prove me wrong.