Fernandes ended his loan spell having played 33 times in all competitions, scoring two goals and registering two assists as Fiorentina also reached the semi-finals of the Italian Cup.
Last week the agent of Fernandes has said the midfielder would be interested in a permanent move to Fiorentina once his loan there ends. Fiorentina have an £8m option to buy in place if a permanent move were to take place.
However, in a new report in the Sun newspaper this evening claims the Switzerland international has agreed terms on a Bundesliga move after three seasons in the Premier League.
The article suggests he is heading to Mainz in an £8m deal.