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The modern full-back is required to do much more than merely defend.
In saying that, Real Madrid certainly need their defenders to have that capacity too; between Arbeloa, Ramos, Pepe and Marcelo their first-choice back four average more than two tackles per game each.
Marcelo makes the fewest in that particular category, and also lags behind his teammates in making interceptions and clearances.
Jose Mourinho could do with an upgrade defensively on that flank, though it is important to acknowledge that these statistics should be taken in context; down the opposite flank Arbeloa might be getting more protection and assistance from Angel Di Maria, for example, than Ronaldo affords Marcelo.
On the other hand, Coentrao consistently rates above Marcelo in these defensive areas—though he has amassed just four league games this season to take as a sample.
Going the other way, ability on the ball is just as important.
Real Madrid look to the left-back to overlap down the flank, widen the pitch in attack and commit opposition full-backs wherever possible. When that happens, Ronaldo is more likely to have a few extra yards of space to operate in and has the possibility to contribute to his team's attack.
Both Marcelo and Coentrao rate at the highest dribbles per game on the squad so far this season, with 1.7 and 1.8 respectively.
Marcelo though, is dispossessed more often than anyone in the squad at an average of 2.8 times per match.
Of course, the physical capability to pull off these aspects of the game week in, week out are also an absolute must. Having a winner's mentality and the experience of playing for a big team is more likely to make an immediate impact for a player.