El Bilal Toure scored the only goal of the game to end Barca's 13-game unbeaten streak in LaLiga and hand the leaders just their second league defeat of the season.
"I am really angry," Xavi said after the match. "It was the worst game of the season from my point of view, above all in the first half. It was a day to make a statement in LaLiga, and there is no excuse. There was no intensity, there was not even a desire to win, and that worries me. We slipped up and we apologise to the fans. If we want to win LaLiga, we have to change chip.
"Luckily we have a seven-point cushion, but we know it's going to be hard to win LaLiga. The first half is the thing that has most disappointed me this season. We showed faith in the second half, but the first half angered me."
The surprise loss caps a disappointing week for Barcelona, who were knocked out of the Europa League on Thursday by Manchester United at Old Trafford.
Things don't get any easier for the Catalans, who travel to face Madrid in the first leg of the Copa del Rey semifinal on Thursday. They will still be without Pedri and Ousmane Dembele -- who both missed the Almeria game with injuries -- at the Santiago Bernabeu, but Ansu Fati could return.
"Of course we will pick ourselves up," Xavi said. "People cannot think we will win LaLiga easily. It is going to be really hard to win trophies. We are competing against the current champion of LaLiga and the Champions League [Madrid]. We are Barca, but we have to play with more intensity and rhythm than this. It was a huge disappointment."
Midfielder Frenkie de Jong said Barca's performance at Almeria should not be attributed to a hangover from the United game or thoughts turning to the Copa Clasico prematurely.
"We have not thought about the Clasico," he told reporters. "LaLiga is very important and the next game is always the most important. ... Things just did not come off, especially in the first half. Now we have to keep going and work hard to improve.
"We are really disappointed. We had the chance to go 10 clear. Now it's still seven but ... we did not expect to lose this game."
It was a first-ever win for Almeria over Barcelona in 16 games, but more importantly, it ended a run of three successive defeats to move them out of the relegation zone.
"The win is what stays with us above all," coach Rubi said. "After three losses in a row, with teams at the bottom getting results elsewhere, we could have ended in the bottom three today. In addition, we had the best team in the league coming here, the leaders, and we managed to keep them out. We worked hard, so hard, not to leave them spaces."
Almeria held Barca to just one shot on target, while they had four, and Rubi said it was a performance to savour.
"When [teams are] crossing, crossing, crossing, I think we are OK," he said. "We didn't suffer in that area too much. I worried more when they played through the lines.
"We worked extraordinarily hard, scored a goal we have worked on and our goalkeeper has hardly had a save to make. Everything came off and it's a day we will remember for a long time."