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The highly decorated left-back played 75 minutes as Real and Chelsea played out a 1-1 draw in Tuesday's first leg.
Following the game, Marcelo issued a rallying cry to his team-mates, telling official club channels: "We have to go [to Stamford Bridge] to win. We have to go for it and play high up in their half."
But, according to El Mundo, Marcelo will not be on the plane to London as the Brazilian, as a Madrid resident, has been randomly picked for local election duties.
It means, instead of preparing with his team-mates, Marcelo will be working at a polling station on May 4, the day before the game, and related Covid travel restrictions will prevent him from joining him team in London.
Marcelo, a Brazil international, obtained Spanish nationality in 2011, a common path for South American footballers in La Liga, where there is a limit on the number of players clubs can have from outside the European Union.
All Spanish nationals on the electoral register are liable to be called up to work at polling stations, for which they are paid 65 euros (£56), and require mitigating circumstances to be excused.
Real Madrid can - and likely will - appeal the decision. Levante were previously successful with an appeal after goalkeeper Aitor Fernandez was picked - and then released - for electoral duty.