The young Madrid midfielder started ahead of Casemiro at the Camp Nou but was replaced by the Brazilian just after the hour mark.
Marcos Llorente had to be substituted during the Copa del Rey semi-final first leg clash between Barcelona and Real Madrid, with concerns now about how long the youngster will be out.
Llorente injured in Copa Clásico
Just after the hour mark, in what was a thrilling game, the Madrid midfielder went down on the grass and didn't take long to ask to be taken off. Apparently, Llorente complained of a muscle injury, on the inside of the left thigh. In his place came the more experienced Casemiro.
This is the second injury in the last month and a half on that area of the leg for the 24-year old. The previous incident, as Real Madrid announced in an official statement on January 1, was when the player suffered a "grade 2 injury to the left adductor."
Tests will be carried out in the coming hours and days to determined how serious this latest injury will be.