The Spaniards have a mountain to climb in the quarter-final second leg following their drubbing in Switzerland, and the Argentine struggled to explain their poor first-half display in his post-match interview.
"I have no explanation of how we went out to play," Pizzi told reporters. "I am 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. At the break all we did was correct what is wrong because we did not do anything at all right in the first half."
Pizzi went on to acknowledge how difficult a task Valencia face in the return leg, but insisted that his players will do "everything possible" to reverse the deficit.