The Argentine playmaker was carried off on a stretcher in the first half of Tuesday's 3-1 Champions League victory at home to Bayer Leverkusen.
The injury is a further blow to Valencia coach Unai Emery after creative midfielder Sergio Canales was last week ruled out for three to six months because of a serious knee injury.
Valencia are third in Group E, one point behind Leverkusen and three adrift of leaders Chelsea with two matches left.
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