Imagine that you are 15 and become the last professional player signed by Matt Busby. Imagine that shortly after your 17th birthday you are called upon to make your Manchester United debut because Denis Law is injured. Imagine that it is against Manchester City at Maine Road in front of a crowd of 63,000. Imagine that the final score is 33 and that the game comes to be regarded as the finest post-war Manchester derby. Imagine that midway through the first half the ball is crossed from the right by Brian Kidd, George Best touches it to you and you score the game's opening goal. Imagine all this and people would say, rightly, that you had some imagination.

No wonder, therefore, that when Sammy McIlroy sits back to recall the formative moments of his playing career he cannot help but use words such as "unreal", "fairytale" and "unbelievable". Everything that is imagined above actually happened to Sammy McIlroy.

Yet even now it seems too farfetched to behold, and if merely a fraction of that story were to be replicated today it would merit newspaper pullouts and television specials. But when he takes his seat at Maine Road this morning for the long-awaited return of the Manchester derby, after an absence of four years, McIlroy will reacquaint himself with the fantastical scenario that became his surreal reality on a bright November day 29 years ago.

That McIlroy spent some of his last playing days in the colours of City, via a three-season stop-off in Stoke, only adds to his already heightened sense of what this morning's fixture means to the people of Manchester. "Everything", is his answer.

McIlroy had some career after that day, staying 13 years with United and playing in two World Cup finals for Northern Ireland. Now, of course, he is Northern Ireland's manager. Having briefly played with Mick McCarthy at City, McIlroy established his managerial reputation by leading Macclesfield Town out of non-League football and into the second division. Two promotions - plus another denied by the Football League: all done with no money. But November 6th, 1971, remains as clear to him as the recent highs at Macclesfield. Maybe his debut day's surprising origins explain his memory's clarity, or maybe it's because such drama is near inconceivable, unforgettable to a 17-year-old from the Newtownards Road in east Belfast.

It began the night before. "There was a lot of talk all week about Denis Law struggling to make the game," McIlroy recalled from his Cheshire home on Thursday. "But on Friday nights then they used to play the mini-derby, United reserves v City reserves. I'd been looking forward to it. Then Bill Foulkes told me I wouldn't be playing. `Report for the first team in the morning, collar and tie.' That was all he said, he didn't go any further. I was disappointed, I was thinking I would just be going along to help Malcolm Musgrove lay the kit out. I was very upset that night."

McIlroy's pain was genuine. He did not ring his parents to say he was included in the first team as he did not think he would be playing. The following morning, as if to emphasise the comparative innocence of the era, McIlroy took the bus from his digs in Stretford to Old Trafford. A United fan at the bus stop recognised him and wished him good luck. McIlroy thought: "What's he on about?

"It had been in some papers that I'd been added to the squad. But I didn't know that. I didn't think I'd even be in the 12."

McIlroy arrived at the ground at 11 o'clock to be told by Frank O'Farrell, the manager who had succeeded Wilf McGuinness who had succeeded Busby, that indeed he would be in the 12, in the starting 11 in fact. Law was not fit, McIlroy was to form part of a forward line with Best and Kidd. Willie Morgan and Bobby Charlton were also in the team - it could be said United were going for the win.

"All week, nothing," he recalled. "Then this. I didn't have time to get nervous. That was obviously their way of doing it. We went for a pre-match meal in Davyhulme Golf Club, then got the coach to Maine Road. "Then you start seeing the fans, red and blue, that's when it sinks in: `You're in.' But people like George Best, Willie Morgan, Tony Dunne, they were great, they didn't make a fuss. They just said: `Go out and play'. That really helped me. "But when we ran out the roar was an unbelievable sound: 63,000, then you know you're in a game. I was 17. Then you look across and you see the likes of Lee, Summerbee, Bell, Doyle. City had a great side then."

A feeling of disbelief remains as McIlroy retold the events. And he was not finished there. "The atmosphere was fantastic. Both sets of fans were making such noise. I can remember the cross. Tony Book was tugging at George's shirt and George couldn't get the ball. It came away from George and I just happened to be following up and put it past Corrigan. I could have run out of the ground on the volume of the noise. Great.

"Maybe I was too young to even think about it. From disappointment on the Friday to 11 o'clock on the Saturday being told you're making your debut. It was just: `Bang!' " There was no champagne or nightclub afterwards. United had a match on the Monday night and Law's fitness remained uncertain. McIlroy went back to his digs. Photographers were waiting, fame had descended. "That was that."

Well, not exactly. Denis Law was unfit for the Monday and when he finally came back Kidd was injured. "I played four on the bounce. I scored four out of four and I scored against Big Pat (Jennings). That was another thrill, against Spurs at home."

The teenage striker was rested for a while, and by the time the most memorable Manchester derby of them all came around 21/2 years later - at Old Trafford, Law's last kick in professional football, for City, relegated United for the first time since 1938 - McIlroy was a central midfield figure for Tommy Docherty.

There he stayed, until one day he had a major argument with Ron Atkinson, who had just bought Bryan Robson and Remi Moses. McIlroy stormed out and a couple of days later he was a Stoke City player. He was only 28 and he still regrets the hastiness of his decision: "Very Newtownards Road. Today your agent would talk you out of it." Players did not have agents then.

And after Stoke came City. Billy McNeill talked McIlroy into returning to Manchester in 1985, but while other players, such as Law and Kidd, had made the change without too much outside discomfort, McIlroy was abused by a section of the City fans who could not forgive his red roots. "I think it was the Last Busby Babe tag that did it."

He featured in only one Manchester derby for City. "Weird", he called it. United won 3-0 at Maine Road. He chatted to Robson and Norman Whiteside in the bar afterwards. They did not talk about the match.

But McIlroy said that his mind could not stop remembering another match. He had given his all for City that day, but though it was the same turf, same teams, same derby, somehow it was not like the first time. But then, how could it be?