Real Madrid have confirmed that Lucas Vazquez suffered a cruciate ligament injury during the Clasico with Barcelona on Saturday.
Vazquez went down after a heavy challenge with Barcelona midfielder Sergio Busquets and had to be replaced late in the first half as his side went on to beat their rivals 2-1.
It has not yet been determined how long the Spain international will be out of action, but he will miss the rest of the season after the diagnosis was confirmed.