Jaime Rodriguez of Spanish outlet El Mundo reported on Wednesday (via The Athletic) that Marcelo was selected to work at a polling location for next Tuesday's Madrid Assembly elections, the same day the club is scheduled to travel to England ahead of Wednesday's rematch with the Blues.
Chelsea and Real Madrid played to a 1-1 draw in the first leg on Tuesday. Marcelo played 77 minutes before being subbed out.
Real Madrid have worked to get Marcelo out of election duty, saying he must travel with the club and can't fly to the match afterward because it would break the club's COVID-19 bubble, but so far the appeals have been denied.
Marcelo, a Brazil native who became a Spanish citizen in 2011, can formally appeal as a professional who "must participate in public events held on the day of voting" that were scheduled before he was alerted of the duty.
Rodriguez noted Levante goalkeeper Aitor Fernandez was freed from election duty on a second appeal in November 2019 and subsequently played in a La Liga match.
Marcelo, who's struggled to earn a full-time role for manager Zinedine Zidane this season, drew the start in the first leg against Chelsea because of an injury to Ferland Mendy, whose availability for next week's second leg is uncertain because of a calf injury.
Even if Mendy is unavailable, Los Blancos may look in a different direction after the 32-year-old left-back struggled with the pace of the Blues' attack in Tuesday's draw.
"They're very quick, have good players and started very well," Zidane told reporters. "In the second half, we had more control. We're still in the tie, and we're happy with the overall performance. We're also good at playing away from home. We press high."
The winner of the Chelsea-Real Madrid tie will take on either Manchester City or Paris Saint-Germain in the 2021 Champions League final. City and PSG play their first leg on Wednesday.