A sub-par Atletico were dispatched 3-0 by Sociedad in San Sebastian, leaving Simeone's side 12 points shy of unbeaten La Liga leaders Barcelona with five fixtures remaining.
Atletico will head into that first-leg encounter in London with three defeats and a draw to show from their previous four road matches in all competitions.
"We have to take a good look at our away form and be more alert and attentive because we are letting ourselves down on the road," Simeone told reporters.
"Today we did not play a good first half. We improved at the start of the second half, but we did not have the consistency to get closer on the scoreboard.
"We have to look at what happened, but congratulate Real Sociedad on their deserved win."
Atleti host fifth-placed Real Betis on Sunday with the chance to close the gap to Barca back to nine points.
However, even a win would not deny Barca the opportunity to close out the title away to Deportivo La Coruna seven days later.