Osasuna pay price for Barca 'mini-crisis'
The Spanish and European champions had been questioned after letting slip winning positions to draw 2-2 at Real Sociedad and at home to AC Milan by the same scoreline in the Champions League over the last week.
"Today they wanted to make sure beyond any doubt and they have put eight past us," Mendilibar told reporters.
"These teams on top of their game, playing at 100 percent, if they don't score eight could score five like they did against Villarreal.
"They have done us over angered by the so-called mini-crisis which was nothing more than draws in games they completely dominated.
"I'm not worried about history books. We play to try and do things well and when you go in at half-time losing 5-0 you start thinking more about the next game.
"What we can't afford to do is lose the next game and look really foolish."
Barca's World Player of the Year Lionel Messi led the scoring with a hat-trick as the side responded to suggestions from some quarters that they had slipped up during the week due to complacency.
Barca coach Pep Guardiola denied he had been angered by their previous two performances.
"I defend my players and up until now they haven't given me reason to be angry," he told reporters. "They might do one day but they haven't yet."
Guardiola played down the significance of the result as did Barca's Argentine midfielder Javier Mascherano.
"We weren't a disaster on Tuesday [against Milan] and we aren't suddenly marvellous now," he said.