Mainoo, 20, has become out of favor at Man United with manager Ruben Amorim who is unable to fit the midfielder in his preferred tactical system.
The English international is supposed to weigh his options with various clubs in the Premier League and interested abroad.
According to the Daily Mail, Napoli would like to sign Mainoo for a first loan with an obligation to buy at the end of the season.
It is also said from Napoli that it is close to the signing of colleague United Rascast Rasmus Hojlund, has been very successful with Scott McTominay.