Real Madrid have mastered the art of winning without hitting top gear this season but that strategy will be severely tested in Wednesday's Champions League last 16 showdown against a Napoli side that have developed their own invincible aura.

La Liga leaders Real were riding high as they set a Spanish record 40-match unbeaten streak in all competitions. That run was finally snapped by Sevilla last month and now Napoli have taken up the mantle of the hardest team to beat in Europe, stringing together 18 games without defeat.

Real's scrappy 3-1 win at basement club Osasuna on Saturday displayed visible weaknesses in their rearguard as Zinedine Zidane's use of a back three bore mixed results, with goalkeeper Keylor Navas called upon numerous times to thwart the La Liga strugglers.

The return of a fit-again Dani Carvajal should see the coach revert to his favoured 4-3-3 line-up as he has an almost full strength side at his disposal except for Gareth Bale, who has recently returned to training following an ankle injury.

Real captain Sergio Ramos said his team needed to underline their credentials on Wednesday ahead of the return leg at Napoli's notoriously atmospheric San Paolo stadium.

"Napoli are strong opponents on a very good run and we'll try and keep a clean sheet so we can have a more comfortable trip over there," the defender said in a broadcast for Facebook Live on Monday.

Napoli striker Arkadiusz Milik, who returned to the squad for Friday's 2-0 win over strugglers Genoa but stayed on the bench after four months out with a knee injury, could make his comeback against Real.

Belgian forward Dries Mertens has taken on the team's attacking responsibilities in Milik's absence, plundering 13 goals in his last 10 games and inspiring a run of seven wins and one draw in 2017, taking Napoli to third in Serie A.

An estimated 10 000 Napoli supporters, including the team's most famous player Diego Maradona, are expected to make the trip to the Spanish capital for the first competitive meeting with Real Madrid since 1987.

In addition, Jose Callejon and Raul Albiol will face their former side for the first time.

"We're feeling the fans' support a lot. The Bernabeu is an intimidating stadium to play at but the only way to get past Real Madrid is to be brave and go there with intent," Napoli and Spain goalkeeper Pepe Reina told radio station Onda Cero.

"Maradona's presence will motivate us and we hope we don't disappoint him."