Based on our system and current squad JoC and Bash are the only player who in my mind are irreplaceable in the short term. They both play such a unique role, which they've been playing for years. To think any replacement would be up to scratch is ambitious, regardless of any fees.
In terms of names mentioned...
Egan has been immense, but is definitely replaceable. Its cruelly simplistic to Egan, but head it/kick it centre halves are in plentiful supply, the kicker with Egan is him as part of a defensive unit.
Fleck again been excellent, but again not irreplaceable if he were to go. On the assumption he goes and that we get a decent fee, this could easily be reinvested into a comparable player - not sure who but its doable. I'll caveat that no one in the current team would be adequate (Freeman, Osborn, Lunny)
Lundstrum, to me is average and could be improved very easily. I preferred Besic, I prefer Osborn, I prefer Berge at RCM, I'd much prefer Swift.
Can't really see any other people attracting interest, maybe the fullbacks?