Teams wear T-shirts with message of support
Both sets of players took to the field at San Mamés on Thursday evening wearing T-shirts with the message "Ánimo, Yeray" ("Stay strong, Yeray").
The 21-year-old, who Athletic have said is making "positive" progress in the wake of his operation, was in the stands to watch his team's 2-1 last-16 first-leg win.
San Mamés crowd in ovation for Yeray
And in minute 27 of the game - chosen because of the player's shirt number - Yeray was on the end of an ovation from the supporters in Bilbao.
Curiously, the gesture happened to coincide with a goal by Athletic's Iñaki Williams, who scored Los Leones' second with the applause underway.