Their Bundesliga opponents were duly dispatched, with the scalps of Liverpool and Boavista to follow, as Martin O’Neill’s squad made it all the way to Seville, where they experienced an agonising but laudable extra-time defeat by Jose Mourinho’s Porto.
Any Celtic supporters dreaming of a reprise of those heady days of 2003, however, should perhaps consider booking themselves in for a reality check.
The Scottish champions may be in rude financial health as they prepare to host Zenit St Petersburg in the first leg of their Europa League last-32 tie tonight but you won’t find manager Brendan Rodgers jumping on any bandwagon of belief that his team are likely to be lining up in Lyon for the final in May.
“I don’t think it will ever go back to what it was 15 years ago,” observed Rodgers. “The game has changed since then and Celtic have changed. Look at the players Celtic had then – they were taking some of the best players from the English Premier League. They were bringing the likes of Chris Sutton, John Hartson and Neil Lennon up here. They were bringing Paul Lambert back from Borussia Dortmund.
“So the financial climate is totally different now to what it was back then for Celtic. There will always be the level of expectation here and we always want to fight, like our predecessors at the club, to do the very best we can in Europe. But when you compare it, football and Celtic have changed a lot.”
Celtic last week announced half-year financial results showing revenue of £71.5 million, a profit of over £17m and around £30m cash in the bank, all a consequence of Rodgers guiding them to the group stage of the Champions League two years in a row.
But he is dismissive of any suggestion those figures should provide the wherewithal for Celtic to build a squad capable of reaching the Europa League final.
“Come on,” he said. “The club is run very, very well. You can spend the money okay, but it’s about the players wanting to come to Scotland. That’s the big factor now. It’s about wages, astroturf pitches up here and all this. There are multiple reasons why a player would or wouldn’t come here.
“What Celtic’s balance sheet show is that the club, the institution, is being run very well. They do spend money but they have to run it sensibly.
“I’m a realist. I’m in my dream job at Celtic, but I’m also a realist. What we have to do year on year is try and somehow bridge the gap in Europe, but it will always be difficult for us.”
That view is supported by the statistics which show Celtic have not won a home match in Europe, outwith the qualifying rounds, in their last 11 attempts. It’s a sequence which saw them lose all three Champions League group games at Celtic Park this season to Paris St Germain, Bayern Munich and Anderlecht.
It makes it hard to argue with Rodgers’ assertion that Zenit, the highest-scoring side in the group stage of the Europa League, are favourites in the tie and why he would happily accept even a 0-0 draw in Glasgow tonight.
“Yeah, any result with a clean sheet for us would be a good result,” added Rodgers. “We’ve seen before that if you can take a clean sheet into the second leg, then it can be difficult for your opponents when they are at home. There is that bit more anxiety and pressure in the tie. We saw that this season when we drew 0-0 at home to Rosenborg then beat them in Norway in the second leg.
“So it’s important for us to defend well against Zenit. That has been the disappointment for us at this level. We have shown that we can defend, but your concentration level has to be so high. Playing domestically and then going into European football you can’t take your eye off anything for a second. You have got to be constantly concentrated at this level because of the level of player you are facing.
“Zenit should be favourites. I never say that lightly but I have always been realistic. When you look at the investment Zenit had made and level of players they have, they will expect to go through. We just have to find a way and we will see if we can get a result.
“It’s always going to be a stretch for us in Europe. What we have to try to do is somehow just edge a way forward and that comes through experience, that comes through being around these games as much as we can.”
If Celtic are to secure a lead to take into the second leg, a return to optimum form for Moussa Dembele would be a significant factor in achieving that aim. The striker has scored just once in his last ten appearances but Rodgers sees signs that he is emerging from a spell disrupted by both injuries and the uncertainty over his future during the recent transfer window.
“He is beyond all of that now and he is back on track and working hard,” he said.
“His focus and concentration is good and when Moussa is at that level, he is real handful and one of the top young strikers about.”