“In January when Chelsea knocked on the door , he was quite convinced , ” Belgian reporter Terreur said.
“But once you are in the south of Italy , with good money and you are a god , why would you change that?
“He would come to the Premier League as a nobody and in Italy he is a star.”
Mertens , 33 , became Napoli’s outright all-time top goalscorer on Saturday
The Belgium international’s contract expires at the end of the season , but he has reportedly agreed a three-year extension despite interest from Inter Milan and Chelsea.
Terreur added that Mertens’ love of London , where he and his wife regularly go for mini-breaks , also made Chelsea’s offer attractive.
Chelsea have made attacking additions this summer , with a deal agreed for Ajax’s Hakim Ziyech , and RB Leipzig striker Timo Werner expected to follow.