Mainoo is seeking to be released on loan before the upcoming transfer deadline, which is on Monday. However, Napoli has also offered him a permanent move to the Italian club.
Mainoo is said to be aware of Napoli's interest and is attracted to the idea of joining the team, especially after the success of his former Manchester United teammate Scott McTominay with Napoli.
Napoli is willing to offer Mainoo a four-year contract, but Manchester United is reportedly seeking a significant transfer fee of £50m for the player.