The turn of the year has coincided with an uncharacteristic dip in form from Zinedine Zidane's Real Madrid.Having enjoyed a 40-match unbeaten run throughout 2016, Los Blancos have won half of their fixtures in the new year, drawing twice and losing twice, while also culminating in their Copa del Rey exit and the end of their treble dreams.Attention for them now turns to recapturing LaLiga and retaining their Champions League trophy, the latter task being a very difficult one, given the calibre of their last 16 opposition.While Real's form in 2017 has been somewhat inconsistent, although injuries have played a big part in that loss of rhythm, that is at odds with the outstanding purple patch Napoli are going through.Coach Maurizio Sarri, formerly an investment banker in Tuscany before turning to football full-time, has been forced to revamp his squad this season after losing former Real Madrid striker and top goalscorer Gonzalo Higuain to Juventus last summer.Replacing the 36-goal haul he managed in Serie A last season was never going to be easy but Sarri has done so in extremely impressive fashion, relying on goals from a number of different individuals in his squad.Their most recent victory, a 7-1 thrashing of Bologna away from home, showcased just how difficult a threat Napoli are going to be for Real Madrid.For the first time in their history, they scored more than five goals in a top flight away fixture and it is the level of their goalscoring that will have Zinedine Zidane worried about how he is to contain them.They have set a new club record for most goals after 23 weeks of the season, currently sitting on a staggering 55, with more in the first half (22) and the second (33) than any other team in the division.During their ruthless demolition of the Felsinei, Marek Hamsik hit his first Napoli hat-trick and Dries Mertens also weighed in with one of his own, taking his tally for the campaign to 16.That kind of continuous creation of chances has caused problems for each side they have faced of late and Real Madrid will likely be no different.In Los Blancos' last 14 games in all competitions, they have kept just three clean sheets and conceded 18 goals during that time.During the same period, the Partenopei have scored 38 times and while there are still concerns over their own defensive solidity, it is by showing a fearlessness and eagerness to attack that will give them their best opportunity of overcoming Real.While there are danger men all over the field who have the ability to hurt the Spanish giants, including Hamsik, Mertens, Lorenzo Insigne, Piotr Zielinski and possibly the returning Arkadiusz Milik, it is arguably a man who is very familiar to Los Merengues who can cause them the most problems.In a team filled with stars and quality in attack, it is often easily to overlook the impact of Jose Callejon but he has been a consistently high level performer and a virtual ever-present for the Campania club ever since arriving from Real in 2013.Following his 10 million euro move from the Estadio Santiago Bernabeu, which looks ever more like a bargain, the Spaniard missed just one league game in his opening season and featured in every single match for the next two consecutive campaigns.Now into his fourth season at the Stadio San Paolo, he is looking ever more comfortable. In fact, after his impressive performance against Bologna, he brought his assist tally for the season to nine, which is more than any other player in Europe's top five leagues.As well as his nine assists, the former Espanyol ace has also contributed with eight goals and could be the man primed to do the most damage to Real Madrid.But whether it is him or any of his more than capable teammates, the holders will have their hands full in trying to keep Napoli quiet and keep hold of their trophy.