A first-half brace from Matias Delgado put the hosts two up at the break and although Valencia improved after the interval, Valentin Stocker's stoppage-time chip put the Swiss outfit in firm control of the tie heading into the second leg in Spain next week.
Pizzi was dismayed at his side's performance, and was at a loss to understand the contrast between their first and second-half showings.
"I have no explanation of how we went out to play," he said.
"I'm ashamed of the way we played in the first half. I do not understand why we didn't play the first half as we did the second.
"Today at the break all we did was correct what is wrong because we did not do anything right in the first half.
"Nobody settles things individually, neither the coach nor the players. We will do everything possible to try and come back in this tie."
