The Spain international was substituted midway through the first half of Tuesday's home game against the Belgian club after colliding with a Saint-Gilloise player and grimacing in pain.
Medical tests confirmed the injury was caused by a hyperextension of the joint.
"Further assessment carried out on Wednesday confirmed a high-grade injury to the posterior capsule and the semimembranosus muscle, but the ligaments and meniscus were not affected," Atletico said in a statement.
Spanish media reported that the 28-year-old could be out for at least a month, which would rule him out of Spain's World Cup qualifier away to Georgia on November 15th and Atletico's Champions League home game against Inter Milan on November 26th.