Barcelona let a late lead slip as they were held to a 2-2 draw at Estadio El Sadar by newly-promoted Osasuna.
The hosts broke the deadlock after less than seven minutes as wideman Brendan Thomas beat Clement Lenglet before clipping a ball across to the unmarked Roberto Torres, the striker rifling his volley past a helpless Marc-Andre ter Stegen.
Ernesto Valverde's men proceeded to dominate possession and dictate play from deep, yet they rarely threatened as Osasuna made it to half-time with their lead still intact. Arguably, Los Rojillos looked likeliest to grab the game's second goal, offering an aerial threat at corners.
As senior players struggled to break down a resolute Osasuna backline, 16-year-old substitute Ansu Fati picked up the gauntlet for Barcelona. Having been brought on at the break, the winger made an instant impact as he guided Carles Perez's cross into the corner of the net.
The comeback was completed less than a quarter of an hour later as Arthur - who had also been brought on at the break - whipped a wonderful effort out of the reach of Ruben in the Osasuna goal.
Barcelona should have wrapped up the win from that position, but they instead had to settle for a point when Gerard Pique blocked a shot with his hand, Torres grabbing a brace by converting the resultant spot-kick.
Key Talking Point
Without Messi in the team, Barça look very beatable. The Argentine is expected to return after next week's international break, but his teammates have been exposed in his absence.
It's never a good sign when a club is hugely dependent on one player, even when that man happens to be a five-time Ballon d'Or winner. Were the 32-year-old's injury struggles to continue into the autumn, an early Champions League exit would not be too surprising.
Before that, however, the Catalans simply must get their domestic campaign back on track or risk having to play catch up with Real Madrid and their city rivals Atletico. Failure to do so would leave their manager staring down the barrel.
Starting XI: ter Stegen (6); Semedo (5), Pique (6), Lenglet (4), Alba (5); Busquets (7), Roberto (5), De Jong (6); Perez (7), Rafinha (5), Griezmann (5).
Substitutes: Fati (7*), Arthur (7), Vidal (5).
STAR MAN - Ansu Fati
The visitors were dire, particularly in the opening half of the encounter. It was looking like they would need something special to avoid another damaging defeat and something special was duly delivered.
Tasked with changing the course of the match, Fati did precisely that with a clinical header. Had Pique not handled late on, his would have been a game-winning performance, instead of just one that spared Barça a second loss in three fixtures.
He and Arthur changed the course of the contest, the pair adding purpose to a lethargic Barcelona lineup. However, the teen just about edges the Brazilian for his impressive display of maturity and confidence at such a tender age.
Fati and Arthur scoring for Barcelona. Two game changers and two substitutes.— Roy Nemer (@RoyNemer) August 31, 2019
Starting XI: Ruben (7); Vidal (6), Hernandez (6), D. Garcia (8), Estupinan (7); Torres (8*), Oier (6), Merida (7), R. Garcia (6); Avila (5), Brandon (7).
Substitutes: Villar (6), Ibanez (7), Moncayola (5).
Roberto Torres from Osasuna could have easily moved to a bigger club in Spain.— EiF (@EiFSoccer) August 31, 2019
Never did, stayed loyal to Osasuna and helped them go back up to first division. El Sadar has always been a brutal away fixture for Barcelona. Not surprised at all that Barcelona are losing early on.
After the international break, Barcelona resume their La Liga campaign at home to Valencia on Saturday 14 September. Los Ches are also competing in this season's Champions League having secured fourth place in the league last term.
Osasuna play the following day away to Real Valladolid, who finished just two places above the relegation zone in 2018/19.