"I'm very sorry for what has happened.
"What we can avoid are certain situations but as of now, we will have to be much more strict. We can't allow these situations to happen."
Coach Xavi Hernandez called the situation a "miscalculation" and said it did not help his side, who suffered their first defeat in 16 matches in all competitions.
"I've tried to tell the players that the important thing is what happens on the pitch, but logically this could have affected us. It's very clear," Xavi told a news conference.
"We've tried to focus on the game and we've not been good. It has nothing to do with one thing and the other. It's not an excuse," he said.
"It hasn't helped us, but on the pitch we haven't been good and we haven't competed."