Chelsea shocked Atletico 2-1 in Madrid for their Champions League clash.
"We had never faced a team with the intensity of Chelsea," Simeone told Cadena Ser.
"We struggled to press them effectively. We improved when (Antoine) Griezmann went on (Tiemoue) Bakayoko and left David Luiz free. But we were imprecise."
When then asked about the quality of English sides in this season's Champions League, Simeone replied: "The English teams have improved so it's much more open. There is less margin for error now."