I think He's a victim for being part of Luca's legacy, and the new head honcho wants to clear all the decks. But I'm left with the impression it's also very significant to see Mattiacci leave Ferrari completely.
Normally a caretaker manager could expect a sideways move back into such a big organisation as FIAT/Ferrari, so something has clearly caused a bigger problem here.
The para below, by Adam Cooper on master.motorsport.com, throws a bit more light on the firing.
Mattiacci fell out of favour with Ferrari chairman Sergio Marchionne, and his handling of the departure of Fernando Alonso – which led to a huge pay-off to the Spanish driver after the team was left with three World Champions under contract for next season – clearly did not help. Just eight months after being chosen by Luca di Montezemolo to run the F1 team, he is out of the company. Also with the appointment of an F1 commission insider is Marchione seeking to turn back time, hoping to reinstate the old
Ferrari
Internationale
Assistance?