The Spanish youth international, on loan from Real Madrid, picked up the injury during Valencia´s 1-1 La Liga draw at home to Athletic Bilbao on Sunday.
The 20-year-old will have exploratory surgery in Madrid and his time on the sidelines will depend on the course of action taken after that.
He has been a key figure for Valencia since joining in the closed season and is set to miss the rest of their games in Champions League Group E, where the side lie third.
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