It was an evening to forget for third-placed Atletico, who had both Mario Suarez and Diego Godin sent off and now trail city rivals Real Madrid by three points.
Simeone's men had challenged Barcelona at the top of the table earlier in the season but, having won just one of their last five matches, they are now 15 points behind the leaders.
And the Atletico coach thinks their recent dip is partly down to the fact they no longer pose a puzzle to opposing teams.
"We went out for the win," Simeone told the club's official website.
"I think in the first half we had more chances. The squad gave everything and we were on top until the sending-off of Mario.
"The guys are working hard. They have had an impressive season and we must continue to respond in the remaining eight fixtures."
Regarding their form, the Argentinian added: "I think we could look at it from the point of view that we have 60 points.
"The season is long and the opposition are beginning to get to know each other and find solutions.
"We won in Pamplona, we drew against Valencia and here. We would like to continue to improve and it is true that we are not as clear as we have been."