"Tests have confirmed that the player has pulled a muscle in his left hamstring," read a statement on Barcelona's website.
"He will be out for approximately five to seven days, and will therefore miss Saturday's league game against Levante and next Tuesday's Champions League match against BATE Borisov."
Gerard Pique will also miss the home meeting with Levante, although the central defender's absence is due to suspension.
Pique appeared to deliberately get himself booked against Rayo in order to be sure of missing the Levante clash and therefore guaranteeing his presence in the Clasico away to Real Madrid on December 10.
However, if found guilty of deliberately forcing the yellow card, the former Manchester United man could be handed an extra one-match ban as well as a 600-euro fine.