Milan soaked up pressure for an hour before striking to win 2-0 with Kevin-Prince Boateng and Sulley Ali Muntari.
“It’s a really bad result,” confessed Pique. “We played badly and cannot make excuses, not for the pitch nor the referee.
“When they scored first, we lost control. This is a difficult place to play and Milan are a club with a big history.
“Obviously we will try to win the second leg, as we must improve and learn from this evening. We will look to respond with one of those magical nights at the Camp Nou.”