I agree, the club have talked before about having a sort of 'modular' approach, where if one person leaves it doesn't bring everything tumbling down. If you know your striker is on his way out you start looking for replacements in readiness, there's no reason why you shouldn't be doing the same for a manager. In some ways it's even more important.
SL was very clear in his interview this week, where DH stood, so I assume he told MA the same thing; therefore, MA should have been compiling a list at that point at the very latest. Knowing MA, we will get Neil Harris...