Barcelona lost 1-0 at Getafe in the Spanish league on Saturday for its first defeat since Koeman took over with the mission of rebuilding his former team, which ended last season without a title.
Barcelona had started this season with two good wins over Villarreal and Celta Vigo. But its limitations in attack were seen in a 1-1 draw with Sevilla in the last round, and Getafe made Barcelona look clueless once the visitors fell behind.
Getafe lived up to its reputation as one of the top defensive sides in Spain and limited the visitors to two scoring opportunities.
Lionel Messi hit the upright in the 20th minute and Antoine Griezmann wasted a great chance on the half-hour mark.
After that, Getafe tightened its marking and Barcelona didn't threaten again until Djene Dakonam almost scored an own-goal when he cleared a cross onto the crossbar.
José Bordalás' Getafe went ahead in the 56th when Jaime Mata converted a penalty kick after Frenkie de Jong fouled Dakonam on the edge of the area while trying to clear a loose ball.
The first loss in four league matches this season for Koeman's team comes before its Champions League opener against Hungarian club Ferencvaros on Tuesday and the season's first "clásico" match against Real Madrid next weekend.
Koeman replaced Quique Setién in August after Barcelona's humiliating 8-2 loss to Bayern in the Champions League quarterfinals. He led an overhaul of the squad to bring in younger players, and the team had made a promising start before the draw with Sevilla and now the more costly slip at Getafe.
"It was a very hotly contested game, with many fouls that never let the match get any rhythm," Koeman said.
After the match Koeman complained about the refereeing, saying "I don't know what we have VAR for" and "there were some dangerous fouls" that went unpunished.
Koeman seemed to be referring to an incident in the first half when Messi went down after Allan Nyom's arm swung into his face.
Koeman also said that he told Bordalás that Nyom "had insulted me and disrespected me, and that he needed to tell his player that that cannot be." Koeman refused to say what Nyom had said to him, only claiming "it was ugly."
Facing important matches ahead, Koeman changed his starting lineup for the first time this season.
Ousmane Dembele, who had only played 20 minutes previously, started in place of Ansu Fati. New signing Pedro "Pedri" González, who had played as a substitute in all four previous matches, started in place of Philippe Coutinho. Fati and Coutinho had played with Spain and Brazil, respectively, during the international break last week.
United States defender Sergiño Dest made his first start since joining Barcelona three weeks ago from Ajax in place of injured left back Jordi Alba.
Koeman also moved Griezmann into the striker role, with Messi moving to the right. But Griezmann's struggles continued despite the switch. He fired his one chance high after Pedri had left him alone against goalkeeper David Soria.
After Mata put Getafe ahead, Koeman sent on Coutinho and Fati for Pedri and Dembele. Getafe's Nyom nullified Fati's incursions and Coutinho suffered from the same suffocating defending as Messi in the middle.
Koeman tossed on forwards Francisco Trincão and Martin Braithwaite for Griezmann and de Jong with 10 minutes left.
Juan Camillo, however, almost grabbed a second goal for Getafe when he got free on the break, only for Neto to get enough of a touch on his shot to send it off the crossbar.
It was Getafe's first home win over Barcelona since 2011.
"We had to fight for every ball," Mata said. "We knew we had to play as a compact group and not make any mistakes. That is what I take away: the hard work of the entire team."
