If they get a lot money for him, then they wont have problem replacing that 90%.
They will, imo. At least in the first season.
They will want another top quality AMC to replace him and they will surely overpay for that new AMC (who may or may not be a success in Italy). They will maybe have money for just another good player.
Milan, if they sell Kaka, will still need at least 4 more top quality players to be a scudetto contender.
Pirlo is done, Gattuso is a ???? after the big injury, Ambrosini is 32, Nesta won't play again, their defenders are all +31 years old, Seedorf plays hot-cold, Ronaldinho too, Pippo is 36 and he plays well for 2 months max. Sheva is history, Borriello can't take them to another level.
Only Pato, and maybe Flamini and Thiago Silva (but MAYBE) can be useful after a year or two. If they lose Kaka, they will need a miracle to make a good team for next year.
Milan are in much deeper shit than we are.