By FOX Sports staff
Following the 0-0 first-leg draw at the Estadio Ciudad de Valencia, Rubin entered Thursday's clash in Russia as slight favourites with home advantage and eventually sealed their progress after extra-time thanks to goals from Jose Rondon and Vladmir Dyadyun.
In doing so, the Russian giants also eliminated Spain's last-remaining representative in the competition and Diop believes the damage had been done a week ago.
"I think we should have won at home in Valencia to set up qualification," he said on UEFA's official website. "But here they scored two goals from the only two chances they had.
"We go home very sad but with our heads held high.
"It was a very difficult match. We knew our opponents would be as defensively strong as we were."
The Senegal international also believed the only thing his side were lacking on Thursday was a touch of fortune.
"They [Rubin] defended very well, as did we," Diop added. "They created bits and pieces but there was not a lot of danger.
"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."
Rubin striker Rondon admitted Levante had proved a tough hurdle to overcome but hailed the "great job" done by his team-mates.
"It was a very difficult match," he said. "We knew Levante had a lot of fight and they would cause us problems.
"It went to extra-time but we were strong physically and we managed to win. I'm happy, we did a great job [and] have to continue like this."
Levante will now turn their attentions to qualifying for Europe once more, although they are currently 10th in La Liga, nine points behind 6th-placed Real Betis.