"Following tests carried out today on Lucas Vazquez by the Real Madrid Medical Services, he has been diagnosed with a sprain to the posterior cruciate ligament in his left knee. His recovery will continue to be assessed," a club statement said.
Madrid travel to Anfield to face Liverpool on Wednesday for their second leg quarterfinal tie with a 3-1 aggregate advantage.
Zinedine Zidane's side then travel to Getafe on Sunday in the hopes of further cementing their challenge for the La Liga title.
The win against Barcelona on Sunday means that Madrid are top of the table on 66 points. However, Atletico Madrid are in second only on goal difference and have a game in hand.