Chelsea go to Spain for the second-leg trailing 3-1 on aggregate.
"We weren't at our top level," Christensen told the club's website.
"We started good for 10 or 15 minutes, good energy with the crowd behind us – and they stayed behind us the whole game – but they successfully took the tempo out of the game.
"We have conceded seven goals now in two games which is not normal for us. It's not just about this game, we did it as well at the weekend. We have to fix it, and we have to do it quick."
Christensen knows they have a mountain to climb but he also pointed out Chelsea are fortunate the away-goal rule is no longer in operation.
"It's two goals, it's still going to be very difficult, but it can't be worse than this. We have to go there and make a big performance.
"We have to try and turn it around but it's going to be difficult. We weren't on top and we know that, but we have nothing to lose so we have to go there, put in a big performance and hopefully turn it around."