I don't necessarily think a new manager has a massive rebuilding job on his hands: this is, after all, the same squad people were talking about as our best since the 80s. Whether or not that was hyperbole, there's talent, youth and experience in this squad. The problem has been a tactical and physical mismanagement that has corroded the psychology of the group. There's certainly a requirement for a few new players but if there's a rebuild needed it's mental rather than recruitment-driven, achieved on the training pitch rather than with the chequebook.
That said, with every drubbing we can only become a less appealing option (to those few managers good enough to have options).
New manager's first year will be spent training and building the setup. There will be some excitement but mostly homework. We'll get the true result in year two. No massive rebuilding is required unless the club dumps a lot of first team players this summer.