Atletico were beaten 2-0 by lowly Espanyol, who were the better side throughout and triumphed thanks to an own goal from Stefan Savic and Leo Baptistao's effort.
The hosts were without the suspended Diego Costa and Angel Correa, while Antoine Griezmann was an unused substitute following his midweek exploits in the Europa League semi-final victory over Arsenal.
Simeone told reporters that he was not surprised to see his side lose their unbeaten home record, saying: "Playing as often as we have played, we knew there could be a game like this.
"We lacked concentration and that comes into play. It is logical, we are short of players and it showed.
"We hope to forget this defeat as soon as possible, clear our minds and to try to finish the league season as best as possible, and face the final of the Europa League as best as possible."
Simeone suggested that some of his team's senior players have been struggling with the congested schedule.
"We are very few. Costa was suspended, Correa was suspended, [Diego] Godin is 34 years old, Gabi is 34 years old," he said.
"We are very few and the final is four days after the next league game."
Midfielder Vitolo was replaced after 26 minutes against Espanyol due to a thigh injury.
Simeone added: "It's a shame about Vito because I wanted continuity and I was getting it.
"Let's hope the injury is as small as possible."