The 23-year-old made 29 Serie A appearances in a loan spell with Fiorentina as they narrowly avoided relegation in the 2018/19 season.
He is yet to play under West Ham manager Manuel Pellegrini and looks set for a London Stadium exit.
The Sun claim Fernandes has agreed terms with Mainz ahead of an £8million move to the German club.
The deal would see the Hammers make a small profit on the midfielder, who joined the club for £5.5million from Sion in 2016.
Earlier this month, Franco Moretti, Fernandes’ agent, said his client had enjoyed his time in Italy.
“The boy is very happy with his experience in Fiorentina, he is very happy to be in Italy,” he said.
“At the end of the season we will meet with the owners and the coach and we will talk about the boy’s future, trying to take the best decision for everyone.
“With the new coach [Vincenzo] Montella there was immediately a good relationship. The boy is very happy.”