In isolation there was nothing special about the goal, almost exactly a year ago, that gave Napoli brief parity in a group-stage tie with Besiktas. It was a sharp enough finish from Dries Mertens, getting across his man to stab in José Callejón’s low centre, but the headline was that they proceeded to lose at San Paolo for the second time in four days. The fact that the Turkish champions had joined Roma in putting three past Napoli at home looked of much greater consequence than a stand-in striker’s demonstration of instinct.

Napoli's perfect start continues after compelling Serie A weekend | Paolo Bandini Read more

Thirty-eight goals later, Mertens, who takes on Manchester City with Napoli in the Champions League on Tuesday night, has been the dominant force in a side whose irresistible form suggests a first Serie A title in 28 years may be a matter of time. Few could have seen this coming from Mertens, previously a capable winger who had an eye for goal but had largely played second fiddle on the left to Lorenzo Insigne since joining from PSV Eindhoven in 2013. A serious injury to Arkadiusz Milik led Maurizio Sarri to test a hunch that Mertens could fill the void up front; the rest is history and, were anything to happen to Mertens now, the gap would more resemble a chasm.

“I never thought he could be a striker like he is now – never thought it,” says Marco Heering, who was the assistant manager at the Dutch club AGOVV Apeldoorn when Mertens spent three formative years there between 2006 and 2009. “I’m very, very surprised. He started in midfield for us and then we used him on the flanks, as he had good actions and could learn to run better with the ball in that space. The way he is playing now is unbelievable, and it’s because he is very smart.”

That spell at AGOVV, who were in the second tier and sought to improve by developing young talents such as Mertens and Nacer Chadli, accelerated a career that was yet to take off as planned. Mertens had not convinced in his native Belgium at Anderlecht or Gent, who both doubted the durability of his 5ft 7in frame. Heering remembers a 19-year-old who, when he arrived on an initial season-long loan, struggled on heavy winter pitches and looked more comfortable refining his evident technique on an artificial surface. He could dribble and pick a pass better than most of his team-mates; now he needed moulding into a team player who could master the game’s dirtier side.

He has got the hang of it. Sarri called Mertens a “little animal” last month and the player compared himself to a “street dog” in an interview earlier this year. Mertens’ hunger is unstinting: many of his goals have been attributable to a sharpness and aggression that confound even the hardest-wired of Italian centre-backs but he has come alive outside the box too, dropping off to participate in buildups that Pep Guardiola believes are unrivalled in Europe.

Although Heering says he and AGOVV’s then-manager, John van den Brom, would “never, never, never, never, never” have contemplated fielding Mertens through the middle, there were a few pointers in hindsight. “After training he would stay and shoot at goal from every distance,” he remembers. “We knew he could play football very well, but you must score too. That’s something he worked at all day, every day. You need that attitude to become the player he is now.”

Napoli have never looked more ready under Sarri to challenge for Serie A title | Paolo Bandini Read more

Mertens was known for being a compulsive trainer, often joining his housemate Chadli to agitate for extra sessions on the pretext that they could otherwise be bouncing off the walls. The fruit of those labours has become clear. Mertens’ range of expression has been breathtaking and the whipped, angled finish over the stranded Lazio goalkeeper, Thomas Strakosha, last month drew instant comparison to a famous goal Diego Maradona scored against the same opponents in 1985. The 30-year-old is not at the old master’s level but if Napoli – yet to drop a point in the league – maintain their momentum then the city’s street artists will be preparing a few fresh murals in six months’ time.

“He’s becoming bigger and bigger and bigger,” says Heering, who is now assistant at Almere City FC. “Normally 25 to 28 is the best time for a player but since his 28th birthday he has become better, and maybe it’s because of the position he plays now in a very good team. The way Napoli plays, so positive and with everything on the ground, is exactly the way he likes it.”

Sarri lamented after Mertens’ flourish against Lazio that his calling had been discovered only recently, saying he “could have been at the top level in the world for years and years”. But football is rarely that simple and it is as well to bask in the present.

Mertens was rested for Napoli’s last Champions League away match, a surprise defeat at Shakhtar Donetsk, but can be expected to play against City even if the club president, Aurelio De Laurentiis, would rather focus on the weekend’s Serie A match with title rivals Internazionale. A decisive statement against Guardiola’s team would suggest the importance of that unremarkable strike in October 2016 is looking greater all the time.