A disappointing afternoon for Barcelona was compounded by potentially serious injury to Gerard Pique, who was forced off following a freak collision with Atletico Madrid star Angel Correa.
Needing a positive result at the Wanda Metropolitano to kick-start their spluttering Liga campaign in 2020-21, Barca instead fell behind just before half-time due to a horrendous mistake from Marc Andres ter Stegen.
The Germany goalkeeper was caught far from his net and rounded by Yannick Carrasco, who was left with an easy finish into the open net to put Atleti in the ascendance.
Worse was to come for the Catalans after the break, when Pique hobbled out of the game in evident pain.
The central defender found himself in Correa's path when the winger was felled by Jordi Alba, and as he tumbled he did so directly on top of Pique.
Pique's knee took the full brunt of the Argentine's weight, and concern around the Barca team was clear as he received treatment on the Wanda Metropolitano turf.
The Spain international was eventually able to take to his feet aided by two Blaugrana assistants but could barely put any weight down on the affected area, which at first glance appeared to be his right knee.
He proceeded to hobble to the sideline, where Sergino Dest was waiting to take his place.
Back on the pitch Barca's malaise continued, rarely threatened to turn round a game that finished 1-0 to the Colchonero and leaves the Catalans marooned in mid-table, with just one Liga victory in their last six outings.
Barca must now hope that Pique merely suffered from the impact of Correa's fall, for which the Atletico player was absolutely blameless, and that he has not suffered lasting damage which could keep him out of action in a key period of the season for Ronald Koeman's charges.
The Catalans are back in action in the Champions League on Tuesday when they visit Dynamo Kyiv, the first of three consecutive midweek Group G prior to December's visit to Ferencvaros and hosting of Juventus.
In La Liga, meanwhile, Koeman's men will return to the field next Sunday at home to Osasuna, as they seek to finally turn around a run of horrendous league form that has made them outsiders to regain the title from Real Madrid by the end of the season.