Real Madrid are blessed with a hugely impressive squad, deep in key areas with top talents and, even in those parts of the pitch where the drop-off is steepest from first-choice to reserve, plenty of versatility to cover if injuries strike.
Even so, there are certain players who offer not just a certain level of quality, but also a way of playing that isn't replicable by any of their potential back-ups, and Marcelo is one such player.
The Brazilian left-back has played just 10 shy of 2,000 minutes already this season, and only three outfielders have racked up more: Cristiano Ronaldo, Luka Modric and Toni Kroos. He's an undisputed starter, a frequent wearer of the captain's armband and an outlet for the team dominating the left flank—but he's also injured with a shoulder problem and struggling to recover before the UEFA Champions League knockout phase starts up again, per Marca.
Real will doubtless give him every chance to recover for the last-16 tie against AS Roma, but in the meantime, there's an opportunity for someone else to prove their worth. The maligned figure of Danilo has to make sure it's him or risk being benched for much of the rest of the season, making a mockery of his €31 million summer move.
Real Madrid are well-known for their extravagant spending on transfer fees and for chasing names rather than tactical necessities, but even so, the summer of 2015 was a new level of interesting choices.
They bought back their own players in Casemiro and Lucas Vazquez, signed a backup goalkeeper but failed to sign the first-choice one they had earmarked in the David de Gea/fax machine fiasco, and they also spent over €60 million on two areas that were arguably their most nailed-down in the entire team outside of Cristiano Ronaldo's spot: central midfield and right-back. Mateo Kovacic has yet to really stamp his authority on the team with Kroos and Modric the absolute choices, while Dani Carvajal had the right-back area locked down prior to Danilo's arrival.
All the while, the opposite side of defence went worse than ignored, with no left-back coming in to challenge or back-up Marcelo and Fabio Coentrao leaving for the season on loan.
It has meant an imbalance throughout the campaign with at least four different players other than Marcelo playing in the role, none of them natural left-backs and achieving varying degrees of success. Left-back will need to be one of the main positions that Real look to fill in the summer, but for now, it's a case of soldiering on with a best-fit.
Early on in the season when Rafa Benitez was in charge, Marcelo was rested against one or two opponents, especially in the Champions League, and it was versatile Spanish defender Nacho who got the nod to fill in on the left at those times. A solid and reliable defender, he is far from the aggressively attacking option that would replicate what Marcelo offers the team and certainly doesn't drive onward and infield into the final third to link play.
However, Nacho was very much a typical Benitez option: a good team player, a rotation option, capable of following tactical orders but not bringing too much more to the table.
He has clocked up over 1,000 minutes of game time this season, but since Zinedine Zidane took over as manager—more than a month ago now—Nacho's total minutes played stands at just four, on as a late sub against Granada after not even making the bench in a couple of matches previously.
The other option would be Alvaro Arbeloa, more used to playing at full-back than Nacho but with extremely limited game time this season—just six appearances all told and, like Nacho, more preoccupied with defensive work than final-third action.
So to Danilo.
Arriving as a big-money signing, he was alternately heralded and criticised as being a progressive player or one who wasn't good enough. As ever, the truth is somewhere in between, but short-sightedness and a demand for 11 compete players in a side leads to unfair appraisal.
Danilo was quite clearly signed for what he brings to the team going forward: he's a powerful runner who surges into space and can cross or cut the ball back on the run—a perfect tactical fit for how Benitez started the season, playing with one of James Rodriguez or Isco on the right flank of attack, who cut infield and left space for the full-back to exploit.
Against the majority of La Liga's weaker sides—like Real Betis and Las Palmas, the latter early on in his Real career—and against the more open top-half teams—like Celta Vigo, another he impressed against—this makes him a perfect outlet for the team, and he can impact in a positive way.
Defensively, though, he has struggled throughout, and there's little doubt Benitez harmed himself, his team and the player when he started Danilo in El Clasico, for example.
Like Nacho, Danilo has barely featured under Zidane, playing just once for 64 minutes at right-back, and even that was due to another player's illness—but Danilo has played this term at left-back, covering for Marcelo a number of times before Christmas. It's not the best fit for the former Porto man, of course, but it provides a route back into the side.
With Real Madrid playing with freedom and an offensive mindset of late, it shouldn't matter too much that Danilo is on the opposite side—against Athletic Bilbao, if he recovers from his own ankle issue, he needs to be confident and aggressive in his on-the-ball play and use his pace to keep the likes of Markel Susaeta at bay.
Danilo needs to get the fans onside fast, as well as his manager, and that has to start by proving he can contribute to the XI in the same way Marcelo does, in both halves of the pitch.
Dani Carvajal lost the battle to start regularly under Benitez, but he has been straight back in the side under Zidane, playing supremely well in four out of the five Liga matches and coming off the bench in the other when he was ill.
The second-choice right-back for the national team, Carvajal has racked up three assists in the last five games as he continually surges forward down the flank, not with as much pace or power as Danilo manages, but certainly with more technique and composure in the final third. That split-second of vision and foresight, allied to his ability to deliver a telling cross, has contributed to the 19 goals Real have scored in five games under Zidane.
He has likely already done enough to see out the season as first-choice on the right, unless injury or a surprising dip in form takes hold, and Marcelo is the guaranteed starter on the opposite flank.
Danilo must change the perception around his own game if he is to break into the XI on anything approaching a regular basis, both with rotations and as substitute to give rests to the starters. He is capable, certainly has time on his side and can offer physical traits that neither Carvajal nor Marcelo can—but without the intelligence and confidence in his game, that means little other than to be used as a counter-attacking outlet.
Athletic will not be easy opponents to beat in La Liga at the weekend, so any impact Danilo has will be viewed positively. A run of key games against big opponents can quickly turn around the fanbase, but he has to take this chance with Marcelo injured.
It'll be a long four months on the bench if not, for a player hailed as a marquee signing only six months previous.