Mertens has an expiring contract and seemed to be on his way out of Napoli, until it emerged last month that a renewal had been agreed.
However, Gazzetta dello Sport claims the 32-year-old has yet to put pen to paper and there is an increasing sense that he will part with the Partenopei.
The newspaper explains the stalemate could stem from the forward’s demand for any fines being donated to charity being rejected by President Aurelio De Laurentiis.
The Belgian was among the Napoli players who went on strike in November, refusing to go on a punitive training retreat that De Laurentiis had organised.
Mertens was also said to be unhappy with Napoli’s decision to lay off several non-playing staff due to coronavirus.