Derek Johnstone scored the winning goal in an Old Firm League Cup final when his friends were comparing conkers in the school playground. He was the 16-year-old boy who became a man, not to mention idol, overnight. The stuff of dreams does not begin to explain the events of October 26, 1970.

Johnstone, who had only turned professional three months earlier, was summoned to the boot room by Jock Wallace, the Rangers assistant, and handed a pair of complimentary tickets. "Go get a good night's sleep, son, you are playing tomorrow." With adrenalin coursing through the teenager's veins, and his mind working overtime, Wallace might have offered Johnstone a couple of sleeping tablets with the match briefs.

His sleep was interrupted by flashes of boyhood fantasy. A premonition, it would transpire. The inviting delivery from the wing, the instinctive burst between Jim Craig and Billy McNeill, and the iconic header.

Tonight, John Fleck will experience a similarly broken rest as he contemplates the possibility of playing in his first Old Firm derby, aged 17. He will do so in a less innocent era and with the benefit of a more thorough developmental process. The remit is the same today as it was nearly 40 years ago. Rangers expect.

"I would have no hesitation whatsoever about playing John Fleck on Sunday," says Johnstone, an authority on the fearlessness of youth and the man who ended Rangers' four-year silverware drought. "To be blunt about it, he is the best option in what has been a problem position. Rangers have tried half-a-dozen players in that position this season and nobody has really worked in there."

Fleck's road to prominence has been painstakingly plotted since he was absorbed into the Murray Park academy by George Adams, his development latterly overseen by Jimmy Sinclair and his team of coaches.

Johnstone prospered in a more innocent age. He signed schoolboy forms in 1968, was given his first professional contract two years later. He scored twice on his debut, a League Cup tie against Cowdenbeath, and his only other appearance before the final was as a substitute against Motherwell.

In contrast, Fleck has flourished against Aberdeen and Dundee United - two of the more prominent teams in the Clydesdale Bank Premier League - and made an auspicious appearance in exalted company in the recent friendly against AC Milan. A derby match at Celtic Park seems a natural progression.

"The great thing about Fleck is that he has the ability to play, no matter the occasion," says Johnstone, whose Rangers career spanned two stints, 210 goals, 14 league and cup winners' medals and the same number of international caps. "He was Rangers' best player against Aberdeen in the first half and then got starved of the ball in the second, which wasn't his fault. It was his moment of imagination that earned Rangers the penalty against Dundee United and he showed great composure and maturity to take the kick himself.

"His situation is different to mine because he has come through the Rangers system for three or four years, whereas I was there straight from school for about eight months before I played. It was all new to me and the excitement carried me through. At that time, I was playing for fun and without fear. You know the importance of an Old Firm derby, especially a cup final, but when you are 16 you walk out in front of a crowd of 100,000 with a smile on your face."

Fleck's presence in the first team has confounded the popular belief that Walter Smith considers first-team football an environment for grown-ups. Barry Ferguson has repeatedly spoken of his frustrations as a youngster at Ibrox during the nine-in-a-row years, where he was regularly overlooked in favour of Ian Ferguson and Stuart McCall.

Wayne Rooney's reputation rocketed only when David Moyes took over from Smith at Everton. Smith, in his defence, points to a host of young conscripts - Lee Robertson, Steven Pressley, Scott Nisbet, Gary McSwegan and John Spencer - all of whom played peripheral parts in perfunctory title wins in the early 1990s.

Smith has not been shy in making the odd selection googly for the Glasgow derby. The visiting support audibly groaned at the presence of Charlie Adam and Daniel Cousin in August, only to change their tune after Rangers romped to an unexpected 4-2 win. Both have since departed but Johnstone believes that Fleck can spring a surprise regardless of the hype.

"If anything, I think the Celtic full-back will have more to worry about than Fleck because he is still an unknown quantity and that unpredictability can give you an advantage," says Johnstone, primed to give his verdict on Fleck in his enduring role as popular pundit on Clyde 1 Superscoreboard. "Andreas Hinkel is stronger coming forward but I don't think he will be given that chance because Fleck has such great energy.

"Rangers didn't have any balance before. All the good work was done down the right side with Kirk Broadfoot and Steven Davis, so it takes the burden off them having somebody creative on the left. He gives everybody a lift because there is a freshness to his game so as well as Rangers finding an exciting talent, his enthusiasm will also rub off on those around him.

"I expect Rangers to play 4-1-4-1 and if Kyle Lafferty plays up on his own, then Fleck's movement from the left will be a big help to him. With Walter in Old Firm games, you expect the unexpected."

After three seasons in Celtic's shadow, Smith is aware of the significance of a win that would take Rangers above their rivals. "This is the third time Rangers have had an opportunity to move ahead of Celtic and they know they might not get a better chance," says Johnstone.

If and when Fleck takes to the field, Johnstone will indulge in a moment's nostalgia. "Big Jock was never a man for individual comments," he recalls of Willie Waddell's lieutenant and eventual successor. "He just said go out and give it your all' but I remember as I walked out he gave me a kick up the backside with his left boot and a wink. He knew I was going to do well for him."

With the championship challenge rejuvenated, Smith will ponder that same investment in youth.