Xavi said there could be "no excuses" for Barcelona's meek 2-1 defeat to Rayo Vallecano, which saw the Blaugrana miss the chance to go 14 points clear at the top of LaLiga.
Barca look destined to end their four-year wait for a league title in the coming weeks, and Real Madrid's 4-2 defeat at Girona on Tuesday gave them the opportunity to pull further clear.
However, goals from Alvaro Garcia and Fran Garcia handed Rayo a deserved victory, with Robert Lewandowski's late volley proving to be a mere consolation.
Barca have only lost three league games all season, but the Blaugrana have now taken just five points from the last 12 available to them in LaLiga, leaving Xavi to rue a missed opportunity.
"We didn't have a good game, we weren't comfortable. They were very intense and aggressive and they didn't let us play," Xavi said.
"We lacked understanding of the game, we abused long passes. There are no excuses, Rayo deserved to win.
"We tried with courage until the end, but they deserved the victory. We have a margin and there is one less game left.
"We have to think about Saturday. The message to the players is that the league has not been won. It costs a lot to win this league and every game away from home is a war.
"This defeat weighs us down and annoys us. Today we failed and we are leaving annoyed because it was a very good opportunity."
Having lost on their last league trip to Rayo in October 2021, Barca have now suffered back-to-back defeats in Vallecas for just the second time in their history, also doing so in December 2002.
Defender Ronald Araujo believed the hosts warranted their win, saying: "Rayo deserved the victory. They played well.
"In the first half we weren't up to it, but we still have a good points advantage. Now we have two home games that we have to win."
Barca welcome Real Betis to Camp Nou in their next game on Saturday, holding an 11-point lead over Madrid at the top of LaLiga and requiring a maximum of 11 more points to clinch the title.