I will tell you what's wrong with that idea...Because there will be no replacement for him on his level, not even close with money we are willing to spend.
Casemiro? Not even near De Jong in terms of quality, not good enough/ready to be starting anchor even in a period of Montos absence.
Generaly I think that we don't need more one dimensional midfielders, we need complete footbalers in midfield who know to pass, create and defend.
Muntari, Noce, Essien all should go before NDJ.
De Jong does not defend, pass, or create, so where is your argument? Again, De Jong would be sold, a legitimate replacement would come in, and Casemiro would be rotational.
Khedira is rumored to be Transfer listed, and

is in for Casemiro.. Khedira would easily raise the level of our mid, hed be able to replace Monto as well as team up with him once Capitanos back
Yeah, that's 25 million at least.
How much would Cigarini cost?
I think he's a realistic target and perfect fit for the regista role. He can both create and defend very well.
Yeah, Cigarini would be an ideal Montolivo replacement, though I'm not sure where that would leave him when Montolivo returns in January. Maybe if he's good Pippo will show some tactical flexibility and play a 4-4-2 or some shit who knows.
See Cigarini as 8-10 M, though there are plenty of co-ownership options/swaps/etc. that can be orchestrated. If Milan does want to sell Paloschi, Atalanta has shown interest, thus they could pretty easily swap Paloschi's 50% for Cigarini's 50%.
That said, I'd prefer keeping Paloschi and getting Cigarini another way, either through swaps or straight cash homie.
Cigarini and Montolivo could also play together, though there would need to be a crazy engine b2b midfielder to help allay the loss of speed.
We already hav too many average players in our squad, especially on defence and midfield. No need adding an average player like cigarini. Rather, we should save all the $oney we can in this window and get a great young midfielder.
You are criminally underrating Cigarini and what he would bring to this team.
And from when is montolivo level any good? Cancer players like him is part of the reason why this team suck so much. After he got promoted to captain he's been the definition of garbage.
Hopefully you don't speak on behalf of all Norwegians