CELTIC had been so focused on an impending cup tie with a team in black and white stripes that they ended up being blindsided by another.

So much has been written and spoken about next month's Champions League tie with Juventus that it was almost as if their domestic chores had become an afterthought. This was the rudest of awakenings for Neil Lennon and his players.

With no Rangers to trouble them, some expected Celtic to stroll to their first treble since 2001. That aspiration evaporated yesterday over the course of 90 insipid minutes on a dreich afternoon and St Mirren – the Paisley Juventus – took advantage of their opponents' frailties to book a place in their second League Cup final in four years. Ultimately they were well worth the victory.

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St Mirren had been given next to no chance. After all, in eight previous meetings they had failed to register as much as a goal. Here, they scored early and finished strongly.

Charlie Mulgrew's late, late strike for Celtic made the contest look closer than it had been as St Mirren were stronger in almost every department. Hearts were immediately installed as favourites for the final but, based on this showing, Danny Lennon's side will fancy their chances on March 17.

There were personal stories of triumph all across the pitch. Esmael Goncalves arrived on loan from Rio Ave in Portugal just last week, and could not have enjoyed a better start, poking Conor Newton's cross beyond Lukasz Zaluska after seven minutes. If he does nothing else in his four months in Scotland the man known as Isma will always be remembered in Paisley for this.

"It's been an incredible day," he beamed. "To have your first game for a new club as a semi-final is exciting enough. To score, to win and make it to the final is unbelievable. I would say we deserved to win. As a team, we performed well. I'm not sure where the confidence came from, but this was a great day for the club and supporters."

Then there was Steven Thompson, the lifelong St Mirren fan. The striker had spoken about emulating his Scottish Cup-winning heroes of 1987 – the last time the Paisley club had won at Hampden before yesterday. His fine strike, sweeping in Marc McAusland's cutback, gave his team a two-goal cushion and was probably when St Mirren first realised they had a genuine chance of winning.

"The moment I scored had to be up there as one of the best of my life," said the striker. "Scoring at Hampden in a semi-final and running to the St Mirren fans – it will take a while for that to sink in. I said before that I fear every trip to Hampden is going to be my last but thankfully I'll get another bite at the cherry. It's an incredible feeling and I just hope I can do it again in the final."

St Mirren hearts would have sunk late in the first half when Gary Hooper equalised, then again when Celtic were awarded a penalty early in the second half when Jim Goodwin was adjudged to have handled in his box. That seemed to be the point when momentum shifted in Celtic's favour but they had not factored in Craig Samson, who had a personal motivation behind his save from Charlie Mulgrew.

"I would like to dedicate the win to my Aunt Marlene," he said. "She's just been diagnosed with cancer and when I saved the penalty kick I was thinking about her. I just wanted to let her know I love her and we will all be there to help her fight this.

"I wasn't really sure who was going to hit their penalty. I'd been watching videos of Celtic penalties the last few days but Charlie's not hit any. So it was just a case of having a wee word with him and trying to put him off and guess where he was going to go."

For Celtic, this turned out to be third time unlucky. Having lost their last two Hampden appearances to Hearts and Kilmarnock, they were widely expected to make amends against a team with a fairly dismal record against them. "We didn't get going," said Joe Ledley. "In the second half we had some opportunities but it was unacceptable."