Diop was frustrated that Rubin managed to score twice despite not having many chances, but said that Levante just lacked the cutting edge required to reach the quarter-finals.
"It was a very difficult match," UEFA.com quotes Diop as saying. "We knew our opponents would be as defensively strong as we were.
"I think we should have won at home in Valencia to set up qualification, but here they scored two goals from the only two chances they had. We go home very sad but with our heads held high.
"They defended very well, as did we. They created bits and pieces, but there was not a lot of danger and in the end it was small details which allowed them to win. What was missing? Luck, efficiency. If we had scored we would have gone through."