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Deadline day came and went, and the door to the arrivals lounge never opened. For a month, Real Madrid had sat quietly despite the speculation for something more, and they would do so again for one more day. Well, almost.

By late evening, the club had announced that one deal had been struck: Denis Cheryshev was going to Valencia on loan. "Real's only winter deal," lamented Marca.

This wasn't at all surprising, though. Just days earlier, club president Florentino Perez had insisted no new signings would be made. It was also true that Madrid's areas of need were few in number, the squad already stacked with talent and depth.

Which realistic targets could take them to another level? Would any get in the first XI? "None" and "no" were probably the answers, and yet still there was a lingering feeling that one minor need had been left dangerously neglected: a back-up left-back.

Back in November, Marca reported that Madrid were monitoring Liverpool's Alberto Moreno, Wolfsburg's Ricardo Rodriguez and Lyon's Samuel Umtiti. By January, both Marca and AS suggested Madrid's interest in signing a new left-back was genuine. The possibility of recalling Fabio Coentrao from his loan spell at Monaco was also floated.

Yet, that door to the arrivals lounge remained shut. Marcelo is on his own.

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For Real Madrid, such a situation is hardly disastrous, but the potential is there for the left-back position to become problematic.

In Marcelo, the club already possesses one of Europe's leading full-backs and, quite possibly, one of its most undervalued players. Full of dynamism, technical quality and attacking instincts, the Brazilian has grown integral to Madrid's fluency and has been a cornerstone in the sides of Carlo Ancelotti, Rafa Benitez and now Zinedine Zidane.

In January, Marca dubbed him "the irreplaceable soldier"; Madrid were never going to find something superior in the winter market. But the lack of cover is an issue.

Often at Real Madrid, the demands placed on a left-back are largely underestimated and might be unsurpassed on the continent. With Cristiano Ronaldo continuing to start from the left wing but rarely playing there—instead preferring to move inside and operate within the width of the penalty area—Madrid's left-back is effectively asked to cover two positions.

In that exhausting role, Marcelo's effectiveness is remarkable. To date this season, only Luka Modric and Toni Kroos have had the ball more for Madrid, per WhoScored.com; no one has made more crosses, dribbles or tackles; only Modric and Sergio Ramos have made more interceptions; even as a member of the back four, Marcelo has two goals and two assists.

The numbers are immense, but so too is the work load. Last season, only Ronaldo and Kroos completed more minutes than Marcelo; this season, the Brazilian is right up there again in that respect, and no one covers more ground than him.

As such, burnout and injury are very possible outcomes in the second half of the season, and when either has happened previously, Madrid have struggled without a natural, left-sided back-up.

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Already this season, we've seen both Danilo and Nacho cover for Marcelo, but the presence of right-sided players on the left flank negatively affects the balance of Madrid's system.

When Madrid lost Marcelo to injury against Paris Saint-Germain, they laboured badly in the second half. Without him against Sevilla, their energy fell away after a bright start. Without him against Eibar, they never really got going at all.

Since his return in mid-December, the change down the left flank has been notable, but gambling on the 27-year-old's ability to cover that side of the pitch on his own for an entire season is a bit like doubling down when a dealer is holding an ace.

There's uncertainty about how long this gamble will have to last, too.

Though Real Madrid's transfer ban has been suspended for now, opening the possibility of signing players in the summer, it's still very possible that the club's appeal will be heard before then. If it is, and if it's rejected, Madrid won't have a summer window to restructure before the ban arrives in the way Barcelona did.

Of course, certainties are non-existent in this situation that's likely to be protracted, but it's possible that Real Madrid's only option for strengthening at left-back in the next 18 months will be to recall Coentrao, which won't be the club's preferred option.

Could they regret not buying when they could, then? Maybe. Maybe not. But the possibility is there.

Follow @TimDCollins