Real Madrid won 3-2 on Wednesday, with Kylian Mbappe scoring twice.
The visitors were underwhelming on the night, but Xabi later said: "These things happen in the Copa.
"It happened to us, I've seen it happen to other teams, even after the 1-3 we conceded that foul, that 2-3, they pressed, they got back into it, we controlled the first half well and not scoring the 0-3 in the chances we had leaves the game open.
"The goal was to advance, and we've already achieved that. That's why I'm leaving satisfied and happy. We've lacked consistency and coherence in our performance during matches, we needed to be more mature and not think that nothing could happen.
"The numbers are the numbers, but Rodry(go) has had important involvement and we need other people to have that effectiveness; we have had chances."