Athletic Club suffered a blow ahead of their Europa League quarter-final with Rangers this week, with key defender Yeray Alvarez set to miss the match.
Rangers welcome the La Liga giants to Glasgow on Thursday desperate to avoid a sixth home defeat in a row following their dismal 2-0 loss to Hibs at the weekend.
But Athletic's preparations suffered a jolt, too, after defensive stalwart Alvarez picked up a muscle injury in training.
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An Athletic statement said: "Yeray has suffered an injury in the adductor muscles of his right leg and is awaiting assessment."
The 30-year-old has made over 250 appearances for his club and has been one of their most consistent players this season.
He missed the second leg of Athletic Bilbao's comeback second-leg win against Roma in the last round because he was sent off in the first leg in Italy.
And he looks certain to miss the trip to Scotland for the last eight showdown, too.
