Xavi had his first taste of defeat as Barcelona boss with Juanmi securing Real Betis a famous 1-0 victory at Camp Nou on Saturday.
Barca defeated Espanyol and Villarreal in their first two LaLiga matches under Xavi, which came either side of a goalless draw at home to Benfica in the Champions League.
They lacked a serious attacking threat against Betis and were punished on the break by Juanmi with 11 minutes remaining.
Ez Abde missed a glorious chance to salvage a point for the Blaugrana, who could find themselves seven points adrift of the top four and 16 off top spot if Atletico Madrid and Real Madrid win later in the day.
Philippe Coutinho sent a tame effort from Jordi Alba's cross towards the near post in the 11th minute and Rui Silva held it at the second attempt.
Gavi had to go off after the ball was kicked against his head by Hector Bellerin, meaning his status for the crunch Champions League clash with Bayern Munich in midweek could now be in doubt.
After Juanmi had a goal disallowed for a narrow offside decision, Xavi sent Frenkie de Jong and Ousmane Dembele on with Barca toiling.
Dembele was lively but could not find the target and Betis put them to the sword with a swift break that finished with Juanmi sliding home a pass from former Barca winger Cristian Tello.
Barca pushed for a winner but Ez Abde blazed their best opportunity over from eight metres with 10 minutes remaining.
Ronald Araujo was deemed to have fouled Andres Guardado despite appealing for a stoppage-time penalty, leaving Xavi to cope with the bitter taste of a first defeat.