Atletico were leading the first-leg 2-3 last night, before Christos Tzolis struck for the hosts on 89 minutes.
Koke said at the final whistle: "We went 2-0 up and couldn't hold onto the lead. It was a rollercoaster ride, just like the whole year. We need to be more focused, especially on set pieces.
"At 2-0, we have to manage the lead well because it's very difficult to win away from home in the Champions League. We know it's a tough ground; only Arsenal has managed to win here. But we have to pay attention to the details. If we take the lead, we need to hold onto it.
"The team went for the game. We had some chances, like Alex's... then they went on the attack, they put a lot of players forward and they equalised."
On the second leg, the midfielder added: "We need our fans, like the other day against Barcelona. When we're all together, especially at home, we're a tough opponent."