Youngster Fermin Lopez rescued Barcelona a 2-2 draw at Real Mallorca on Tuesday which extended his team's La Liga lead, even though the champions dropped two points.
Barcelona came from behind twice to secure a draw, Vedat Muriqi opening the scoring for the hosts early on, before Raphinha pulled them level.
However Abdon Prats sent Mallorca ahead again and 20-year-old Lopez levelled with a firm drive on his second appearance for the club.
Girona and Real Madrid, second and third, face Villarreal and Las Palmas respectively on Wednesday.
"These careless mistakes cannot happen again," Xavi told DAZN.
"We have to improve. Last year we were so solid and concentrated a lot in defence.
"Today we were not bad but these are two mistakes which should not be happening -- because of these errors we lost two points today."
With Sevilla visiting on Friday in La Liga and next Tuesday's Champions League clash at Porto on the horizon, Xavi decided to shuffle his pack.
The coach benched Robert Lewandowski for the first time, with Ferran Torres playing up front, and gave defender Inigo Martinez his first start alongside fit-again Ronald Araujo.
Pedri and Frenkie de Jong's injury absences left the coach with little chance of rotating in midfield.
Javier Aguirre's Mallorca have not started the season well but knew just how to dig in and frustrate the hosts at Son Moix, with Rafa Benitez's Celta Vigo giving them the blueprint last weekend, despite Barca's late comeback win.
The hosts went ahead after eight minutes through target man Muriqi, who finished from Antonio Sanchez's cut-back, after Barcelona goalkeeper Marc-Andre ter Stegen gave the ball away in his own box with a poor pass.
Joao Felix and Torres narrowly failed to reach dangerous Raphinha crosses, before the Brazilian winger took things into his own hands to level.
The former Leeds United forward drilled home from outside the area to pull Barcelona back into the game.
However the Catalans quickly fell behind again, coming undone all too easily. Abdon Prats ran on to a flick-on from a goal kick and slid an effort past the onrushing Ter Stegen.
- 'Making it too easy' -
"The game was blocked up for us from the start, we suffered more counter attacks than we wanted," Barca midfielder Oriol Romeu told DAZN.
"We've had two games in a row which we're not happy with and we have to improve, we're making it too easy (for our opponents)."
Barcelona started the second half at a quicker pace and Xavi soon sent on veteran striker Lewandowski as well as youngsters Lopez and Lamine Yamal.
The latter duo made a swift impact, with Yamal tumbling in the box under a challenge from Jose Copete, although no penalty was given.
However soon 16-year-old starlet Yamal fed Raphinha, who cut the ball back for Lopez to smash home to level to clinch a point that could prove crucial in the long run.
"We played against a great team but when you fight and you scrap, luck is on your side," Muriqi told DAZN.
"I think that the result is fair."
Earlier Sevilla thumped bottom-of-the-table Almeria 5-1 in one of their best performances under coach Jose Luis Mendilibar.
After a poor start to the season, this emphatic victory provisionally raises the Andalusians to mid-table.
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