Bayern Munich coach Carlo Ancelotti felt his side paid for a sluggish performance in their 1-0 Champions League defeat to Atletico Madrid on Wednesday.
The Bundesliga title-holders had won all eight of their matches so far this season before Wednesday, but a solitary strike from Yannick Ferreira Carrasco sealed a deserved win for the home side at the Vicente Calderon.
Bayern created few meaningful chances despite dominating possession, and Ancelotti was left frustrated by their slow use of the ball and a lack of "bite" when trying to win it back.
"We controlled the game, but the bottom line is that today we had no bite in the tackle, we weren't determined enough. We lost too many balls and played too slow," he said.
"It was a tough game, as we expected. We could have played better. We lost some balls and got hit on the counter.
"They scored and then I think we tried to get back into it but we were too slow, and that's why I do not think we had too many chances to get back into it.
"Losing is always sad. You never want to lose. We need to evaluate the game, look at what we did well and did not do well and go forward. We are at the start of the season. We still have the chance to top the group and we will need to beat Atletico in the return game to do so.
"Against Atletico, you have to be faster and take your chances. It was not a good performance, but it was just a game. Next time, it will be better."
The victory was Atletico's second against Bayern this year, following their 1-0 win in the first leg of last season's Champions League semi-final in the Spanish capital.
Arjen Robben, who came on as a second-half substitute, has called on his team-mates to take on board the lessons of those two defeats ahead of Saturday's Bundesliga match against Cologne.
"This was a big game for the group stage. It was the semi-finals last season, and we saw why," said the Netherlands international.
"We started quite well, we were aggressive and we had chances - you need to take them in a game like this, just as Atletico did. We must learn from it. On Saturday, we want to win again."
Atleti now hold a three-point lead over Bayern at the top of Group D after their two victories to date, with Rostov and PSV further behind on a point apiece following their 2-2 draw.