The 24-year-old Brazilian did not appear to have picked up a serious injury when he was substituted during Real's 2-0 victory over Espanyol.
However, he was left out of the team that recorded a 1-1 draw with Villarreal on Wednesday night.
Today, the Spanish giants confirmed the extent of Casemiro's injury on their official website in a statement, which read: "Following tests carried out today on Casemiro at the Sanitas La Moraleja University Hospital, he has been diagnosed with a fracture to the central third of his left fibula. His recovery will continue to be assessed."
Local reports had claimed that Casemiro, who played in five of Madrid's seven games so far this season, could be sidelined for three weeks.