"After the tests carried out today, our player Eduardo Camavinga has been diagnosed with a sprain of the internal collateral ligament in his left knee," a club statement said.
His injury happened during a training session on Tuesday at Warsaw's National Stadium where the Super Cup game against Italian side Atalanta takes place later on Wednesday.
Camavinga, who featured in four games for France at EURO 2024, is expected to miss up to seven weeks and Real's LaLiga title defence begins on Sunday at Mallorca.