Kylian Mbappé is expected to be out for three weeks with a thigh injury, a source has told ESPN, meaning he will miss Real Madrid's derby with Atlético Madrid this weekend.

Star forward Mbappé was substituted in the 80th minute of Madrid's 3-2 LaLiga win over Alavés at the Bernabéu -- having earlier put Madrid up 2-0 -- after he complained of discomfort in his left leg.

Coach Carlo Ancelotti described it as a "knock" after the match, saying Mbappé "asked to be substituted to avoid any problems," but Madrid confirmed the France international's injury on Wednesday.

"Following the tests carried out today on our player Kylian Mbappé by the Real Madrid Medical Services, he has been diagnosed with an injury to the femoral biceps of his left leg," the club said.

A source told ESPN Mbappé would be absent for three weeks, which would also see him miss games with Lille in the Champions League and Villarreal in LaLiga and the international break with France.

Madrid head into Sunday's derby with Atlético at the Metropolitano Stadium second in LaLiga, with 17 points from seven games, trailing leaders Barcelona. Atlético have 12 points from six games, and travel to Celta Vigo on Thursday before Sunday's derby.

Mbappé's form has steadily improved throughout the season and he has now scored five times in LaLiga, with his best goal of the campaign coming against Alavés.