Mertens has been pushed in to the centre of Maurizio Sarri's attack and has delivered with impressive results.
With Poland striker Milik not due to return from a knee ligament injury until January, Mertens has been pushed in to the centre of Maurizio Sarri's attack and has delivered with impressive results.
Sarri is expect to hand the 29-year-old Belgium midfielder more goal-scoring tasks at the San Paolo on Wednesday when a win for the hosts would ease the pressure ahead of a likely Group B decider away to Benfica next month.
Mertens, expected to get the nod over Manolo Gabbiadini after he returned from an injury knock only in midweek, said: "We'll see who gets picked, but if it's me I will go out and do my job.
"I like this position, I'm playing a lot, doing my job as a centre-forward and learning new things every day."
Standing only 1.69 metres tall, nearly 20 cm shorter than towering centre-forward Milik, Mertens is comparatively harder to find in a packed area.
But that has not stopped the former PSV Eindhoven star from stepping up to the plate with three goals from four group games.
Although Milik hit a brace in a 2-1 away win over Dinamo at the start of the competition, Mertens stole the show with a brace of goals in a 4-2 win over Benfica in late September when Milik and Marek Hamsik were also on target.
After Milik suffered injury playing for Poland in a World Cup qualifier, Sarri's decision to push Mertens into the striker's role quickly bore fruit.
Mertens levelled at home to Besiktas last time out, although a late strike from Vincent Aboubakar secured a shock 3-2 away win for the Turkish giants - who held Napoli 1-1 in Istanbul last time out -- that complicated Napoli's group campaign.
Napoli sit second but on the same points (7) as Benfica, who are away to Besiktas on Wednesday. Besiktas sit third at one point further behind.
Ukrainian giants Dinamo have only one point from four games but Sarri is taking nothing for granted: "We have a clear objective, tomorrow we have to go out there and win.
"Even though the decisive game for us will come in Portugal, this is a crucial game for us. I want us to focus exclusively on our performance and not to worry about the result, or else we risk seeing nerves creep in.
"Dinamo's position in the table doesn't tell the whole story. Since July they've only lost one away game. In Istanbul they drew and against Benfica lost only because they missed a penalty.
"They're solid, strong and have good technique. Tomorrow we expect a much harder game than what people might be expecting."
Asked if he would be happy with a second-place finish for Napoli, which could see a tougher draw for the Italians in the knockout round, Mertens added: "First place is better, but I want to make the next round and I don't care how we do it."