The 59-year-old is widely expected to take over from Antonio Conte at Stamford Bridge imminently, with the Blues now trying to negotiate compensation packages for both managers.
The former Napoli manager is poised to succeed his fellow Italian at Stamford Bridge as he failed to deliver Champions League football after finishing in fifth place last season.
Napoli defender Kalidou Koulibaly has revealed that Maurizio Sarri will be Chelsea's coach next season.
Sarri will bring Blues legend Gianfranco Zola with him as his No 2 once the club have agreed a compensation deal worth £9.5m with Conte and his staff.
"They (Chelsea) can expect nice football because he is a very good coach", said Koulibaly.
"It's disappointing that he leaves like this".
Koulibaly has been linked with a move to Stamford Bridge in the past, and it sounds like he could be persuaded to follow Sarri to Chelsea this summer.
"I can only speak on behalf of my client and reaffirm that there has not yet been any meeting with the Chelsea management".
The polish on Conte's tenure at Chelsea has become tarnished both at boardroom level and amongst his players.
Mertens was one of the best players for Napoli last season as the Serie A side finished second in the league.
The centre-half was the subject of a summer-long pursuit by Chelsea in 2016, and was again wanted 12 months ago, but he has suggested he will be staying at Napoli to work with Sarri's successor Ancelotti.