Now they need to do the same again when NK Maribor come to Celtic Park. But, in what is the biggest 90 or 120 minutes of their season, it might be even tougher because of the circumstances.
As things stand, they are in the draw for the group stage.
However, while that will be the attitude held by the supporters after the 1-1 draw in Slovenia, Ronny Deila and his players can't afford to think that way.
It would leave them wide open to a huge fall. So, the temptation to go back to the system and line-up favoured by Ronny in previous games must be resisted.
Celtic are actually caught in a bit of a dilemma in this regard because the call will be for them to go for it.
But, even though the game is at Celtic Park, they must treat it precisely like the away leg and stick by the five in midfield.
Again, that will be tough on Kris Commons as he would be left on the bench.
But, big games require big calls. That was a big one on Wednesday, and it will be a big one on Tuesday.
Maribor are a decent counter-attacking side, and Celtic going for it just because the game is in front of their fans would play right into their hands.
The emphasis must be on preventing them getting the away goal to cancel out the one scored by Callum McGregor in Maribor.
Once again, look to take a grip of the midfield, keep it compact when you lose possession, and work hard to win the ball back as quickly as possible to counter their counter, if you like.
That might not make for the most exciting football, though Wednesday's game had enough in it to keep everyone glued.
But this is all about getting through. The record books will not describe how you achieved that.
If things are not going the way you need them to, then Kris can be brought on and the system tweaked.
Let's hope it is yet another memorable night for the club, back in their own ground where so many great occasions have taken place.
It is important Celtic start to make it a fortress once again after some disappointing performances and results on European duty.
The best night in the last campaign came when they defeated Shakhter Karagandy 3-0 to overcome the 2-0 deficit from their play-off first leg.
They are much better placed this time around after the draw in Slovenia. But no one should think this is over, not even for a second.
Because that's how long it can take for this to be turned right around.
That would be a huge blow after they have bounced back from the defeats by Legia to put themselves in such a strong position.
It was clear in Maribor that important lessons have been learned, and that fitness has been improved.
It was also clear that, despite having sold Fraser Forster since then - so often a key player in European matches - the side is stronger.
Craig Gordon is looking like the Craig Gordon of old, and huge credit must be given to him for not only retaining his mental strength after so long out injured, but getting back to his best form in such a short period of time.
The addition of Jason Denayer is also a big plus.
I was asked when he made such a good debut in the win over Dundee United last weekend whether I thought the 19-year-old should be pitched into such a big game as the one in Maribor.
I had no hesitation in saying yes because you can see the boy's ability, and his attitude, are top drawer.
The partnership with Virgil van Dijk - who himself is still only 23, remember - is strengthening every day.
It would be a pity if it was pulled apart before we had the chance to see how good it could become by the sale of Virgil.
English clubs are closing in as the transfer window prepares to close.
But, if Celtic go into these last few days with a Champions League place in the bag, it could make it much easier to repel offers and persuade Virgil to stay the extra season which I believe would be good for him in the long term.
Celtic are also stepping up their attempts to add more players, with winger Wakaso Mubarak waiting for a work permit to complete his loan move from Rubin Kazan.
The position I really want to see filled, though, is up front. And it's good to hear the club are moving for Serbian striker Stefan Scepovic.
I'd have preferred the deal to have been done in time for him to take part in the play-offs, but we don't know how difficult it has been behind the scenes.
Going on previous experience, what we do know is that there is likely to be an increase in transfer activity - both in and out - in the last few days of the window.
By then, Celtic will know if they have Champions League money to spend.
Based on what I have seen in the past week, I believe they will.
But, for now, the attitude has got to be that they still have it all to do before that cheque can be cashed.