Sociedad spoil Atletico's perfect home record
Improving Real Sociedad beat them 1-0 to become the first visiting side to take points at the Calderon in the league this season, ending a run of 13 straight home victories for the Europa League holders.
"Although we lacked accuracy we played with ambition but we were unable to hurt our opponents," Argentine Simeone, who last week tied himself to Atletico with a contract extension until 2017, told a news conference.
"Tomorrow morning, we will look to improve where we need to improve as a team. This way, we will get back to experiencing what we have done up until now."
Xabi Prieto's 53rd-minute strike for Sociedad, which television replays showed came from an offside position, was the first La Liga goal Courtois had conceded in front of his own fans since October 28.
"It is a record for the whole team, not only for Courtois," the Belgium international, on loan from Chelsea, told reporters.
"I am pleased because it is always nice to set a record, especially one like this, but it is the work of all the players, not only me. I would have preferred to win and not have the record."
Courtois, 20, broke the previous club best for not conceding at home in the league, set by former Atletico goalkeeper and coach Abel Resino, of 800 minutes in the 1990/91 season.
The defeat pushed Atletico down to third in the standings, one point behind their city rivals Real Madrid, whom they meet in the King's Cup final on May 17.
Jose Mourinho's side won 2-1 at Celta Vigo to go second, 13 points behind leaders Barcelona.