The rugged defender attempted a first-half header at goal in the Liga encounter, and was inadvertently kicked in the face by Atleti defender Lucas Hernandez.
The referee chose not to award a penalty, and Ramos subsequently had his nose plugged up by Real's physios, before being replaced by Nacho Fernandez at half-time.
And it has now been confirmed that the Real captain sustained a fracture of the nasal septum, though it remains to be seen if he will be out for any period of time.
Speaking afterwards Zidane said: "It is a fracture, for Sergio. I believe it's broken. I don't know how long he'll be out for."
Emilio Butragueno, Madrid's public relations director, told beIN Sports: "Ramos is going to have tests but if he has left the game it is a bad sign.
"We have to be careful and see what the doctors tell us."