Marcelo was forced off injured against Sevilla on Wednesday night, making him another injury doubt for head coach Julen Lopetegui ahead of the crucial Madrid derby on Saturday.
The left-back pulled up abruptly in the 72nd minute with a non-contact injury and hobbled off the field of play.
The Brazilian’s premature departure left Real Madrid down to 10 men for the final stages of their 3-0 defeat to Sevilla at the Estadio Ramon Sanchez Pizjuan, having just replaced Luka Modric with Dani Ceballos in their final change of the match.
"Marcelo seems to have a [muscle tear] but we'll have to wait for tests tomorrow," Lopetegui said after the match, while the defender himself confirmed that the injury was to his calf muscle.
Andre Silva hit twice and Wissam Ben Yedder notched a third in the first half as Los Blancos suffered their first defeat of the season and missed out on the opportunity to go top of the table as Barcelona lost 2-1 to lowly Leganes.
And with Isco a doubt for El Derbi as well after recently having surgery on his appendix which forced him to miss the trip to Sevilla, Lopetegui faces the unenviable task of trying to bounce back from a demoralising defeat in a derby without two of his key players.
However, Atletico head coach Diego Simeone said he was ‘optimistic’ about the defender’s chances of being fit for the derby. Isco and Marcelo may also recover in time, but the early signs do not look promising.
And assuming both players are set for a spell on the sidelines, Lopetegui will have to find solutions for the Champions League tie away against CSKA Moscow as well. The former Spain head coach will also want to build momentum heading into his first El Clasico at the end of October, and will be expecting maximum points from the four games that separate the two derbies.
In the absence of a natural replacement for Marcelo, Nacho Fernandez is the most likely candidate to fill in at left-back, although more comfortable in the centre or on the right of a back four.