Xavi Hernandez has said Barcelona's players felt like they were "robbed in their own home" after around 30,000 Eintracht Frankfurt fans gained entry to Camp Nou for Thursday's Europa League quarterfinal.
"The players did not feel like they were playing at home," Xavi said ahead of Monday's LaLiga game against Cadiz.
"It's not an excuse, but the atmosphere conditioned us. We were not comfortable. I had a bad feeling from the first moment when we got on the bus at the hotel and we did not feel at home.
"They were throwing everything at us, [the players] couldn't even get down the tunnel. Then we gave a penalty away in the first minute. It was a fateful night in every sense, sporting and institutionally. It started badly and ended badly. You feel robbed in your own home."
Barca gave Frankfurt 5,000 tickets for the game officially, but many more were able to purchase seats in the home end, with upwards of 30,000 German fans thought to have been among the 79,000 crowd.
President Joan Laporta has said Barca will make tickets for non-domestic games non-transferrable from now on, but Xavi still wants an explanation from the club as to how they effectively gave up home advantage for such a big game.
"The club will give an explanation at some point -- or at least find solutions so it does not happen again," Xavi added.
"It was the club's mistake, for sure. When the club finds out what errors led to Thursday's events, they will explain it to us."
Meanwhile, the fallout among Barca fans has continued with one supporters' group announcing they will not attend Monday's game in protest.
In a statement, La Grada d'Animacio -- who fill the section behind one of the Camp Nou goals -- describe Thursday's events as an "invasion" and a "humiliation" and say they will stay away as a result.
The ticket fiasco has, in part, covered up a poor run of performances. Barca clicked into top gear before March's international break with a 4-0 win over Real Madrid but, following a narrow win over Sevilla, they have struggled in two games against Frankfurt and Levante.
"There were good moments against Levante and in the first leg against Frankfurt," Xavi said. "In general, though, we have lost a lot of balls in the last three games and it has affected us.
"But we have to keep believing in what we are doing. The level has dropped, that's clear, but it's a process. A change in playing style requires time, patience, conviction and hard work. We must get back to what we did when we beat Madrid and Napoli and the results will come.
"We have to improve, we have to concentrate from the start. It's key to score the first goal, it makes teams open up. The project is just getting started. There are good signs, but it's clear we need to improve."
Since Xavi took over, Barca have risen from ninth to second in LaLiga and Thursday's loss to Frankfurt was their first defeat in normal time in any competition since Dec. 8.
They have won seven games in a row in LaLiga and, if Madrid slip up at Sevilla on Sunday, they could move to within six points of the league leaders if they win their games in hand.
"If we don't win the eight games we have left, no," Xavi said of Barca's title hopes. "Before Madrid's game at Sevilla, we have to focus on ourselves. The first objective is to qualify for the Champions League next season.
"If after that we can compete for LaLiga, perfect. It is possible to win all eight games, but it will be tough. Cadiz are playing for their lives, Real Sociedad want to get into Europe, Rayo still need points to survive... everyone is playing for something."
Barca will be without the injured Gerard Pique and the suspended Ronald Araujo for Cadiz's visit, leaving them short in defence, while the game comes too soon for Ansu Fati's return from a hamstring problem.
Midfielder Pedri is also absent with a thigh problem picked up in the Frankfurt loss and is likely to miss the rest of the season.
"Pedri is hurting," Xavi said. "He was in his best form of the season. It's a setback for him and the team."