Christian Pander hit the winner for Schalke, but Aguirre was upbeat afterwards, declaring: "There's still 90 minutes to play. The second-leg will be very different and will be played in different circumstances.
"We'll play to our people and at the Vicente Calderon. We are confident that we can win."
Antonio Lopez saw red for Atletico and Aguirre added: "You know I never comment on referees. We need to learn, to think and be clever in this game. We must stop ending with a man less when playing in Europe. The same happened against Bolton last season."