Valdes, 32, underwent surgery in Germany after tearing the anterior cruciate ligament during a 3-0 win over Celta Vigo on March 26.
Valdes, who is out of contract at the end of the season, will miss the remainder of the season for Barcelona and the 2014 FIFA World Cup for Spain before trying to return in time for the start of next season.
Barcelona president Josep Maria Bartomeu has said the club was prepared to offer the long-serving goalkeeper a contract extension despite reports German goalkeeper Marc-Andre ter Stegen will join the team this summer.
Valdes has made 535 appearances for the Spanish champions, with six La Liga (Spanish first division) titles and three UEFA Champions League titles among his successes.
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