United took control of the tie with a 3-0 first-leg win on Basque soil.
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Afterwards, Yeray said: "It's been a hard blow, we came in with spirits high.
"But we still have 90 minutes left, we have confidence, and we need to go there to make the comeback and try to turn it around.
"The word 'confidence' is what we need to focus on, and go there with the greatest of hopes."
