Celtic proved that they can come from behind but they need to take some of their league mentality into their European tie and believe they are the ones who should be winning, who should be taking the lead and who should be the ones in the ascendancy.
Forgetting the away goals for a moment, if Celtic can win by a single goal, they will go through and taking the lead will go a very long way in helping them to do that.
That being said, it would not be the end of the world if they fell behind given things would not change for them too much – a goal is a must. End of.