Argentine footballer Luciano Sanchez could be out of action for up to a year after his left knee was entirely dislocated in a chilling injury the likes of which his doctor said he has never seen.

The injury took place after Fluminense's Marcelo accidentally stepped with his full weight on the Argentinos Juniors defender's shin, bending his leg entirely backward and prompting gasps of horror on and off the field.

"In 23 years as a doctor, I have never seen anything like this, it is almost a separation of the femur and fibula," Argentino's doctor Alejandro Ronconi told D Sports Radio on Wednesday.

He said Sanchez, 29, would undergo several surgeries to reconstruct the knee, and "recovery takes between 10 and 12 months."

The injury took place at 55 minutes, and the former Real Madrid player Marcelo broke down in tears as he was sent off.

"Today I had to experience a very difficult moment on the field. I accidentally injured a professional colleague. I wish you the best recovery possible," Marcelo wrote on social media.

Argentinos responded: "We are rivals, not enemies," and thanked Marcelo and Fluminense for their concern.

The match drew 1-1 and the two teams will face off again next week in Rio de Janeiro.

The annual Copa Libertadores is the highest-level football competition in South America.