Takefusa Kubo and a double from Orri Oksarsson were the goalscorers for the hosts.
Imanol later said, "By the 80th minute we should have scored more goals than 1-0, given what we saw on the pitch. Then what changed is that we got it right.
"I told you I was calm before the game, because the players were doing a great job and the public appreciates that, because that's why 32,000 of them came and encouraged us from the first minute."
Imanol was particularly pleased with Sergio Gomez's performance: "He came from City, everything has been said, I am not going to discover him now. Without having done preseason, I have put him in various positions and I have asked him for different things, because he has a lot of talent, quality and commitment.
"It is a luxury to have players like Sergio, especially if he is well accompanied, which I understand he is at Real."