Amazingly, McGeady will be the most experienced player on the park tomorrow when Glasgow's big two collide, having first sampled the unique atmosphere as an 18-year-old in May 2004.
Rangers' storming form through December has opened up a seven-point lead over Tony Mowbray's inconsistent Celic side, with the prospect of a 10-point deficit unbearable for the Hoops' support.
On-form McGeady will carry the hopes of the home fans into the showdown, with McStay impressed by what he has witnessed recently from his former youth-team prodigy.
After quitting Celtic during the summer to take charge of Ujpest Dozsa, the former Sligo Rovers boss has been enjoying the winter break at home in Glasgow and catching up on all things Celtic.
He said: "It's surprising to think that Aiden will be the most experienced player in the team. Celtic need to ignite in this match and there's no better game than an Old Firm match to produce your best.
"I've been very impressed with what I've seen from Aiden over the last month, there has been an end product to his game and he's hurting teams.
"No one has ever doubted his ability but he's scored a few goals this season and has had a lot of assists recently as well.
"With Celtic's match being postponed at Kilmarnock, Rangers have gone further ahead in the SPL but playing Rangers at home is always a must-win match for Celtic.
"They are at their best in situations like this, Rangers allow teams to come at them so it will be an interesting match.
"It will take some individual brilliance to win the points and someone like Aiden can swing it Celtic's way.
"I think that he has regained his confidence this season, he knows how he can influence a game and is using that for the team.
"I've mainly been watching Celtic on television this season and his workrate has been high. Seeing Aiden in the flesh again he looks to be back towards his best.
"He's playing with freedom, he's scored a few goals and recently he has provided a lot of assists. The final product is definitely hurting the opposition."
McStay added: "No one has ever doubted Aiden's ability but he's needed an end product to his play and that's coming; the whole team is benefiting.
"Two seasons ago when he was voted Players' Player of the Year he was absolutely magnificent, he could change a game. I've watched him and worked with him since he was a kid but I'd happily pay money to watch him in that sort of form.
"There are other very talented players in Scotland but Aiden has the unexpected about him, he's one of the players that determines games.
"It's important that he is contributing to victories, working hard on every part of his game and is exciting for supporters to watch. He's looking like he's back at his best."
McStay was in charge of Celtic's youth development as McGeady was nursed through the club to make a scoring debut away to Hearts as a 17-year-old.
At that age McGeady expected to be a regular in Martin O'Neill's all-conquering side at the tail-end of Henrik Larsson's career, rubbing shoulders with Chris Sutton, John Hartson, Stilian Petrov, Neil Lennon and others.
He had to wait until Gordon Strachan's second season in charge of Celtic to become a first-team regular but that relationship was always strained and confrontational.
Twelve months ago McGeady spent the New Year at home serving a two-week ban after a dressing room bust-up with Strachan.
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That relationship was unconvincingly patched up but the arrival of Tony Mowbray has given the 23-year-old's Celtic career a fresh lift.
McStay added: "Looking back, the incidents with Gordon Strachan has only made Aiden stronger.
"The rights and wrongs are for other people to discuss, but when you bounce back from something like that to produce the sort of form that he's on just now, you have to admire his strength of character. Aiden has always had a streak in him, I had to punish him at times while trying to shape him into a player that can be a top professional.
"He's came through his problems with Gordon, a player with lesser self-belief and less character might not have survived that."
McStay will enjoy tomorrow's match before returning to Budapest to plan Ujpest's title challenge in the second half of the campaign.
He happily admits to "having the bug again" for management after ending his spell in charge of Sligo to take up the youth team job at Celtic in 1994. Retaining his extensive Irish contacts he had Seamus Coleman over on trial at Celtic a year ago before Everton moved in to snap up the full-back.
Tomorrow he'll resume his role as a lifelong Celtic fan, hoping that Aiden McGeady can play a decisive role in restoring Celtic's title challenge.
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