Barcelona suffered its second straight loss in La Liga as Luis Suarez saw his Camp Nou debut spoiled.
Luis Enrique was stunned by his former side as Joaquin Larrivey gave Celta Vigo a shock 1-0 win over Barcelona on Saturday.
The Barca coach — who arrived at Camp Nou from Celta in the offseason after the ill-fated Gerardo Martino reign — had been looking to get his side back to winning ways after their 3-1 Clasico humbling to Real Madrid last weekend.
However, despite dominating throughout, the Catalans were ultimately left to rue missed chances as Larrivey struck in the second half to send them crashing to another potentially costly defeat.
Luis Enrique handed a home debut to Luis Suarez following the completion of his much-publicized ban and although the Uruguayan was generally quiet, Barca still created a host of opportunities.
The Catalans hit the crossbar four times — twice each for both Neymar and Lionel Messi, the latter of whom continues to chase a record-tying 251st La Liga goal.
Celta, by contrast, was clinical and after a mix-up in the Barcelona defense, Larrivey slotting home in the 55th minute to settle matters.
Despite Barcelona's late pressure, Celta held on to claim a memorable victory, leaving the early title tussle at the top of La Liga wide open.
Barca began the match brightly as Neymar came close twice inside the opening 15 minutes, first heading straight at Sergio Alvarez before rattling the crossbar with an audacious effort from 25 yards.
The hosts failed to build on that early momentum, however, and gave Celta a period of respite, but they soon burst into life again in the 30th minute, with Messi unsurprisingly proving pivotal.
The Argentine forced Sergio into a fine acrobatic save after collecting a well-placed cutback from Neymar. Messi then saw a goalbound effort knocked onto the crossbar by Jonny Castro.
Barca's playmaker came close for a third time in the 34th minute, heading Suarez's left-wing cross over despite being unmarked.
Celta did well to frustrate its hosts, but it was Sergio who continued to be the hero for Eduardo Berizzo's men, denying both Messi and Neymar as the first half drew to a close.
Barca started the second half in determined fashion and hit the crossbar for the third time just two minutes into the period, as Neymar met Jordi Alba's well-placed pass.
And Celta soon capitalized — Larrivey pouncing on uncertainty in the home defense after he had linked up with Nolito to slot past Claudio Bravo.
Luis Enrique's side almost found itself two goals down with 12 minutes to play, as Bravo palmed away a Nolito effort and substitute Charles headed the rebound over from 8 yards.
Messi then hit the woodwork again with a long-range free kick, but Celta held on.