Needing to overcome a 3-0 aggregate deficit, Athletic may have to do without midfielder Oscar de Marcos and star winger Nico Williams.
Both players are struggling with injury and coach Ernesto Valverde, speaking ahead of their 0-0 draw with Real Sociedad on Sunday, stated: "We have to see how De Marcos is after the other day and Nico, who won't be able to play because he's not well, having had this groin pain for a while.
"He's making an effort to be there but he's not well. He's feeling the effects of the season.
"He finished the other day very badly, yesterday he was also bad, today too, and he won't be able to play tomorrow."
Uncertainty around Nico
On Nico, Valverde also said: "(Oihan) Sancet is doing well and as for Nico, we have to evaluate him day by day.
"He's been making an effort to be here but it's been a while now and during the season it doesn't come without a hitch for everyone, all these games so close together.
"They always lead to problems, not just for us. You see Inter, Barça, after so much effort.
"We're not exempt from that happening to us.
"We'll see day by day to see if we can recover and manage well until the end of the season."