The Portuguese forward, who joined Atletico from Benfica in a €126million deal last summer, has endured a difficult debut season at the Wanda Metropolitano, missing 10 games through injury already.
"Joao Felix has suffered a low grade medial collateral ligament left knee sprain, as confirmed by tests he underwent at the Clínica Universidad de Navarra," Atletico said on their official website on Monday.
No time frame for his recovery was given.
"The Portuguese international is pending response to treatment," Atleti added.
The 20-year-old, who has scored six goals in 28 appearances in 2019-20, is suspended for Atletico's next match (away to Athletic Club) anyway.
He should be back in time for the Rojiblancos' trip to Osasuna in the next round of matches, which is scheduled to take place in mid-June as LaLiga aims to get back under way on the 11th.