The pair are Belgium teammates and good friends.
Sportmediaset says Mertens has been urging Fellaini to move to Napoli this summer, bombarding his friend with texts and emails talking up life in Naples.
Napoli are locked in talks with United over a deal for the midfielder.
Rafa Benitez's Serie A outfit would prefer an initial loan arrangement with the option of a permanent deal next June. However, United are pushing for a straight cash sale in the range of £15 million.
Fellaini moved to United last summer from Everton for £27.5 million.
For the big stories and the banter follow us on Twitter: @tribalfootball