Tests carried out on Monday had shown Soldado had a "significant strain" in the thigh muscle in his left leg," Valencia said in a statement on their website.
"He will not be available for the next game," the statement said, adding that the length of time he would be sidelined would depend on his recovery.
Real won last week's first leg at the Bernabeu 2-0 to close on a last-four clash with holders Barcelona or Malaga, who drew their first leg 2-2 at the Nou Camp and play the return game on Thursday at Malaga's Rosaleda stadium.
Jose Mourinho's Real side inflicted a 5-0 drubbing on Valencia at the Mestalla on Sunday, the club's worst ever home defeat.
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