Xavi has revealed Barcelona's efforts in El Clasico last week affected the players' performances in their 1-0 victory over Real Sociedad on Saturday night.
A superb goalkeeping display from Marc-Andre ter Stegen and Ronald Aruajo's 92nd-minute winner helped Barca earn their eighth La Liga win of the season, taking them within four points of current leaders Girona before Real Madrid play on Sunday.
The current Spanish champions were holding on for much of the encounter at Reale Arena and Xavi admitted the display was not good enough.
He said: "We cannot play this football. We entered the match with zero intensity. If we play like this in the Champions League, we will not compete. The first half is completely unacceptable for us.
"Last week we deserved to win and we lost. This week we didn't deserve to win but we did. That is football.
"I'm very happy because the Clasico affected us more than I thought. The first 25 minutes of today are unacceptable. But this is football and it's a golden victory.
"We have to be real. We were not good and we did not deserve to win."
- LISTEN TO THE LATEST EPISODE OF 90MIN'S TALKING TRANSFERS PODCAST: IS TEN HAG'S JOB UNDER THREAT? DE BRUYNE FUTURE & CHELSEA EYE RAMSDALE
Barcelona's rythym has been stunted by injuries to key personnel so far this season but Xavi has welcomed back a number of stars in recent weeks.
Robert Lewandowski was somewhat ineffective in a 3-4-1-2 formation alongside Joao Felix in attack and was brought off before the hour mark as Pedri made his first appearance since August from the bench.
Defender Jules Kounde also started, having not played for the Blaugrana since the 2-2 draw with Granada in October.
Barca's next test is a Champions League clash with Shakhtar Donetsk as they look to earn their fourth win from as many group stage games.