Giuseppe Pondrelli has also been recruited as fitness coach, while Fabio Corradini is Watford's new analyst and Roberto Miggiano becomes a scout.
Following the departure of Mazzarri's predecessor Quique Sanchez Flores, it has also been confirmed that Dean Austin, Alberto Giraldez and Antonio Diaz Carlavilla - who each worked under the Spaniard - have also left.
Frustalupi, incidentally, has worked with Mazzarri for almost 15 years at Inter Milan, Napoli, Sampdoria, Reggina, Livorno and Pistoiese.
Nitti once played alongside the Italian, and has coached under him since 2005.
Vigiani also worked with the head coach at both Inter and Napoli
He previously played for him at Livorno and Reggina.
Source : PA
Source: PA