Barca will now look to bounce back when they face Espanyol in the Catalan derby in La Liga on Saturday. After that, they will prepare for the second leg at the Wanda Metropolitano next week.
Discussing a potential comeback against Atleti, Flick said: "We believe in ourselves because we played very well in the second half with one less player. They also played very well; they have quality up front. The truth is, it wasn't easy to defend against them, but we had chances to win this tie."