The 26-year-old man, who has not been named, was taken to hospital but was released a short time later.
It was reported the man was enjoying a drink with fellow Reds supporters in a bar in the southern Italian city when masked thugs appeared from nowhere.
Police were called to the scene just after midnight, but the attackers had already fled.
Italian daily Il Mattino wrote that groups of young, hardcore Napoli fans had been travelling around the city on scooters looking to fight “hooligans”.
A Naples police spokesperson said that authorities are investigating, using CCTV footage to try and track down the attackers.
“The incident last night was completely unprompted and unmotivated,” Naples police chief Antonio De Iesu said on Wednesday.
“These sort of things do the city no favours.”
Liverpool fans who travelled to Naples for the game were advised not to wear club colours and to avoid public transport to reach Stadio San Paolo.
Reds supporters were provided dedicated transport services to make their way to and from the stadium.
A booze ban was also enforced between 7:00 pm and midnight on Wednesday.