Barcelona president Joan Laporta has assured the club's supporters there will be an explanation for the massive presence of away fans, as the home side was knocked out of the competition after the 3-2 home loss, which followed a 1-1 first leg draw in Germany.
A massive contingent of Eintracht fans witnessed and celebrated after the second-leg win, rubbing salt in the wound for the Blaugrana.
Despite the 5000 ticket allowance, which was as per UEFA regulations, reports from the ground suggested figures of Eintracht fans neared five times that number.
Alluding to the sale of tickets on the secondary market, Laporta, who started his second term as the embattled club's president in 2021, apologised for the situation and vowed there will not be a repeat.
"What has happened in the stands is a shame that cannot be repeated, we must process information and we have to take action, but it is unfortunate," Laporta said.
"We will have to be stricter and not allow certain things, I feel ashamed as a Barcelona fan. I feel embarrassed with what I saw in the stands, I apologise.
"It was very serious. We are processing all the information, and we will give explanations. What happened today is not usual and should not have happened."
Enric Masip, Laporta's advisor at Barcelona, voiced his anger at the situation, which reportedly caused fans at the stadium to protest.
"Everyone has the right to sell their tickets but the reality is that seeing a Camp Nou with so many opposition fans is very, very bad," he posted on Twitter.
After its eliminations from the UEFA Champions League and Copa Del Rey, and with a 12-point deficit to Real Madrid in the league, Barcelona's exit from the UEFA Europa League will likely leave it without a trophy this season.
Laporta believes the club needs to reinforce its identity as a result.
"If it is a failure then we will learn from our mistakes, this defeat should make us believe more in our ideas," he said.
"We haven't been able to win, we've lost the tie and we have to accept it, we're all sad and angry but we have to keep going and fight for the league."