I wouldn't get too hung up on the Pirlo replacement. Players with Pirlo specific characteristics are rare in the modern game in any case. Marrone and Appelt are young players which have some characteristics in common with Pirlo. They are players for the future still (still need a lot of work although Marrone has good character already).
If we really do need a temporary vice Pirlo, the answer would be Pizzaro as he has lots of experience and can be had for a reasonably cheap price.
Nainngolan seems more like an allrounder.
To be honest, as it stands today we have: Vidal, Pirlo, Marchisio, Marrone, and Pazienza and have Giaccherini who can play in Marchisio's spot fairly well. I don't think there is a huge rush to go out and find a Pirlo replacement because if that player is ONLY a pirlo replacement, he won't get any playing time unless Pirlo is injured. Best to use some all rounders and groom players like Appelt and Morrone to grow into that role if need be.