La Real stunned the Old Trafford crowd on Thursday night.
Imanol later said: "Surely there will be very few happier than me, the whole team, staff and fans will be just as happy.
“We knew that we had a very powerful rival, but if we played our game we were going to have our chances. You had to have a bit of luck, but you have to do a lot of things.
"I'm happy with everything we have tried to do, I preferred to lose, but being ourselves and with personality. Today everything has turned out for the best, it's the first game of the group stage and we'll think about the next one.
I knew that Sorloth was going to give us a lot and changing the system and that we could adapt better with Álex.
"With Silva in the lineup you have a lot of chances to win. David Silva now has seven victories on this field, which I venture to say that nobody is going to win eight games."