AS THE mercury hit the mid-90s in unseasonably boiling temperatures, it was more than just the sun which scorched Celtic; the 2003 UEFA Cup final in Seville still smoulders for former Celtic manager Martin O’Neill.

Almost fifteen years on from that May date and his immediate observations from the night remain unchanged; “I don’t care what anyone says, we should never have lost it.”

It was the season that promised everything. A clean domestic sweep, a European trophy. It would be wrong to claim that Celtic ended up entirely empty-handed for although they lost out at the final hurdle on a four-pronged silverware assault, they won a credibility and respect on the continent that had been posted missing long before O’Neill’s arrival at the club.

Glasgow Times: Jonathan Gould was keeper on the day that Martin O'Neill won his first trophy as Celtic manager

But still, it rankles. Twice O’Neill watched as Celtic came from behind against Jose Mourinho’s Porto only to lose to an injury-time Derlei goal after Hoops defender Bobo Balde had been dismissed.

“I have never watched the whole game back,” he said. “It is still too hard to watch. I have watched sections of it back, certain minutes, but I have never sat and watched it from start to finish. Of course, it is still there, the sense of frustration, the sense of injustice.

PART ONE: Martin O'Neill on trebles and testimonials as he looks towards his Celtic park return.

“People will tell me that over the piece we were beaten by the better team but I am not having any of that. We should have won that game and against a team, the bulk of whom were still there, when they went on to win the Champions League the following season.

“Henrik Larsson was magnificent that night. He really was quite outstanding. But he was not alone. We had a number of players out there whom I always felt just never deserved to be on the losing side given what we put into that game and that season.”

For the hordes of Celtic fans who descended on the Moorish town of Seville there was a sense of just how special the occasion itself was. The game was the third European final in the club’s history and now, in the current climate of ever-widening financial chasms, it is difficult to envisage that there will ever be another.

Glasgow Times: Martin O'Neill thanks the Celtic fans for their support following their UEFA Cup final defeat by Porto

A huge part of the reason why Brendan Rodgers was brought to Celtic was to get them back into the elite playground of the UEFA Champions League group stages. He has successfully led Celtic through the perilous route of qualification games and into the lucrative group stages at both times of asking but once there it has been a bruising experience.

The gulf in quality and resources has been particularly sobering as any of the players who trudged off the pitch in the Nou Camp following the harrowing 7-0 defeat to Barcelona would testify, as would those who endured the 7-1 walloping dished out by PSG this season.

PART ONE: Martin O'Neill on trebles and testimonials as he looks towards his Celtic park return.

For a short time they live in the same group but realistically few would quibble that they belong to different worlds.

Bridging a way into it is something that O’Neill believes will be a perennial challenge for any Celtic manager.

“I know it was a difficult season in terms of the Champions League but it is so, so tough when you are in that environment,” he said.

“Celtic are doing fantastically well at the minute and they have dominated domestically but I think what everyone craves is that European respectability. You want to have that credibility in the best tournament on offer in club football but it is not an easy thing to do by any stretch of the imagination.

PART ONE: Martin O'Neill on trebles and testimonials as he looks towards his Celtic park return.

“But a good series of results and performances in the group and it works wonders. You sense the belief in the players, in the crowd, you feel the expectations rise and top players really respond to that.”

O’Neill’s European style was far more pragmatic than Rodgers’. In the time that has passed since O’Neill was bounding along Celtic’s touchline, socks tucked into tracksuit bottoms, relentlessly cajoling his side, the gap between the haves and have-nots has become dispiritingly wide.

Glasgow Times: Brendan Rodgers says he will be speaking to former Celtic bosses, including Neil Lennon and Martin O'Neill

The club’s history and relationship with European football will ensure that there is always a desire to hold court in that arena, no matter the challenges it offers. This summer Celtic fans will look out the Gaviscon to line the stomachs as they brace themselves for four rounds of qualifiers, a journey more gruelling than the group stage itself.

“Crucially, you always – always – need just a little bit of luck,” said O’Neill.

PART ONE: Martin O'Neill on trebles and testimonials as he looks towards his Celtic park return.

“I remember some our matches in the Champions League over the years. We were a minute away from qualifying for the knockout stages in Lyon when big Bobo [Balde] handled ridiculously and we went out because of a Juninho penalty that came with the last kick of the ball.

“Even that first time in the Champions League when we had nine points but still didn’t go through. The margins are so desperately fine at that level.

“My first challenge in Europe was to get Celtic through a qualifier just to get into the old UEFA Cup.

“The next big thing for us was going on and winning the Treble that season and as I said it was an amazing thing to achieve given what had gone on the season before. But to go from that to putting Ajax out in a Champions League play-off just 12 months down the line was an incredible jump.

“We brought in quality players and players who could cope with the pressure and intensity of those games. But also, you need just a little bit of luck along the way.”

PART ONE: Martin O'Neill on trebles and testimonials as he looks towards his Celtic park return.