Simeone said that he took a risk at 1-0 in order for Atletico to grab an equaliser by sending goalkeeper Sergio Asenjo up for a corner, but it backfired to leave his side with an uphill task to qualify for the last 16.
"It is absolutely my fault," AS quotes Simeone as saying. "I thought the game was over [at 1-0] and at 1-1 we would have had more chance [so I sent Asenjo up for a goal].
"Football is a game of risk. This time it went wrong and forces us to score two in the second leg.
"We are lacking precision at the moment and defensively we were poor. In a long season sometimes bad spells happen, but we will work doubly hard to improve."
Madrid travel to Real Valladolid on Sunday.