Diego Simeone accused Atletico Madrid of effectively conceding the first half in their 2-0 loss to Eibar on Saturday.
Atleti suffered only their third La Liga defeat of the season at Ipurua, Esteban Burgos scoring after just 10 minutes before Edu Exposito made the points safe heading into second-half injury time.
It was the first time Atleti had lost a competitive game against Eibar since a meeting in the second tier in 2001, and it leaves them eight points behind league leaders Real Madrid.
Simeone felt they were punished for a slow start to the game, a habit that gave him great concern in the autumn and one he hopes does not continue in 2020.
"We started badly. We didn't play in the first half. In the second, we were okay we played a good game, with more chances for danger," he told Movistar.
"We lost a half. Back in October, we were losing halves and we've gone back to doing that. It's my responsibility.
"It's a very difficult pitch. We've won here 1-0 or 2-1. Today was a similar game. They take advantage of our inattentiveness in the first half and we could have scored in the second.
"We didn't control the game, nor was there any great dominance of the opposition, but we didn't play. They played as ever, with intensity, really clear ideas - the concepts a team needs. I congratulate them."
Saul Niguez, who played at left-back, also admitted concern about the way Atleti too often start matches slowly.
"We struggled a lot at the start. They created three chances and got the goal from the third," he said.
"There have been many times this season when we haven't started games well.
"I think the end of the first half and the beginning of the second were good, but they defended well. We didn't manage to create clear chances despite getting to the byline several times.
"After the good run we were on, it's a tough blow."
Atletico travel to Cultural Leonesa in the last 32 of the Copa del Rey on Thursday, before a home match with Leganes next weekend.
Following that, Simeone's side begin February with a trip to Real Madrid, looking to avenge their recent penalty shoot-out defeat in the Supercopa final.