Valencia head coach Nuno Espírito Santo has slammed his team's performance in their 1-0 defeat to Gent at the Ghelamco Arena on Wednesday.
Los Che could have qualified for the knockout stages with victory over the Belgian champions, but slumped to defeat after Sven Kums scored from the penalty spot early in the second half.
"We're ashamed of our performance," he said. "It wasn't a good game, and I'd go as far as to say that it was a very bad one.
"The only [good] thing is that we can still be among Europe's 16 best teams if we improve enough."
The result leaves the Spanish side second in group H on six points, well behind group leaders Zenit St Petersburg but still two points ahead of Gent.
They face the Russian club next before taking on French side Lyon in their final group game.