The late strike sent a raucous San Mames wild and was enough to see out the win for Athletic Club, who earned revenge for the Supercopa de Espana defeat against Los Blancos only a few weeks ago.
"We're very happy and proud of this team and their work," Marcelino said at his post-match press conference.
“It's very difficult to knock out Barça and Madrid. Now we have to enjoy this. It was a special night that once again become magical. You've seen what these players have done, supported by an amazing crowd. I can only thank everyone because they are the ones who truly make these moments special."
The manager praised his side's performances, reiterating that nobody can begrudge Athletic for their win.
He said: “We played a complete game. We were intense, active and got into their half a lot. We did an immense job. It's very difficult to dominate Real Madrid like we did and we received a well-deserved prize. I'm happy with the effort and the way we played. We were incredible."