Janssen spent one season in London after signing from AZ Alkmaar for £17 million, but he scored just two goals and was regarded as a major bust.
He has fared little better in Turkey with two goals and two assists in 10 Super Lig appearances so far, and there have been rumours he will stay with the club beyond this season.
In an interview with A Spor (via Sport Witness' Nihat Emre Kocaaslan), he remained coy on talk of a move:
“We'll see, for me now it is important to have a good season with Fenerbahce, that's the only aim right now and after that I'll look towards what's going to happen.
[…] “I have my thoughts about this of course, but for now I think it's best to focus on what is happening right now. When we get close to the end of the season we'll see what's going to happen but it's for sure that I really enjoy my time at Fenerbahce.”
Kocaaslan noted Fenerbahce would be unlikely to pay anything close to the fee Spurs shelled out to AZ for in 2016, which complicates a move.
Napoli aren't short of attacking options, as Dries Mertens has emerged as a vital player since his switch to the central areas. Arkadiusz Milik also profiles as a future star, although two severe injuries in quick succession have put his career in jeopardy.
Mertens' contract does include a release clause of just £26 million, however, which could tempt several Premier League clubs, per Calciomercato.com's Lorenzo Bettoni.
Should the Belgian leave Naples―he's not expected to―the Partenopei will likely have to dip into the transfer market for a replacement.
Janssen would be an odd signing for Napoli, who have mostly focused on lesser-known prospects they can develop. The Dutchman still has tremendous athletic upside, and perhaps the Partenopei believe they can unlock his full potential and get him back on track.
Spurs are likely to cash in on Janssen next summer and won't care too much which club ends up signing him, as long as they can recoup some of their investment.