With Barca currently top of the table and eight points ahead of their rivals, victory at the Bernabeu would take them 11 points clear.
That's assuming both sides win the same amount of points between now and then, of course, but it's entirely possible.
A win for Real, on the other hand, would haul them back into the title race.
History suggests the game could go either way, with Barcelona winning three of the last five games against Real on away turf.
Much will depend on their form heading into the titanic clash but according to Marca, Real *could* end up being at a disadvantage, despite being at home.
Los Blancos host Segunda Division B side Fuenlabrada in the Copa del Rey on Tuesday night and given they have a 2-0 aggregate lead, it seems a foregone conclusion that they'll progress.
Real chiefs fear a lot of their fans won't bother going to the game as a result, potentially leaving the Bernabeu rather empty - but they have a plan to prevent this.
Real have warned their four largest supporters groups - the Primavera Blanca, La Clasica, Veterans RMCF and North Fans RMCF - that if they don't attend, they will be banned from the Barcelona game.
A statement read: "By the choice of the club, we have expressly communicated to all fan groups that voluntary cession of your ticket for this match will result in the compulsory cession of your ticket for the Real Madrid-Barcelona match on December 23."
Season ticket holders in the designated fans section are encouraged to pass on their tickets if they can't make it, but Marca claim this isn't possible for cup ties.
This means that Real fans must attend the Fuenlabrada game unless they want to risk being banned from next month's El Clasico. Brutal.
Real Madrid vs Fuenlabrada isn't the most glamorous fixture, but if it means being able to watch the El Clasico, Madridistas would be wise to bite the bullet and go.
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