The American fight fraternity has, with characteristic ruthlessness, dismissed the affair as a nonsense, a risk- free payday for the champion. Even here, submerged beneath the ballyhoo surrounding Frank Bruno's Saturday night engagement with Carl Williams, it has scarcely merited more than the raising of a quizzical eyebrow.
It is tempting to subscribe to the American belief that the fight is a grotesque, cynically made mismatch. After all, everyone knows Nunn, the former middleweight champion who rates amongst the leading dozen fighters in the world. In global terms Ashley, the British light-heavyweight title-holder, is a virtual nobody.
Nonentity or not, Ashley insists that on Friday night a 28-year-old former apprentice bricklayer from Leeds will produce the biggest upset in British boxing since another rank outsider, John H Stracey, shattered the legendary Jose Napoles to win the world welterweight crown in 1975.
'I have always known I was going to be a world champion, it was only a question of when,' he said. 'That question has been answered now. If Michael Nunn gets in the ring with me on Friday night, the title's mine.'
When you have finished laughing, consider these facts. Nunn's most productive days were when he campaigned at middleweight in the 1980s. Now 30, and with a recent whispered history of personal difficulties, his career threatened to disintegrate after losing his title in May 1991 to another unfancied underdog, James Toney.
Since then the tall, fast-moving champion has won his second title, his revival sustained against carefully selected opponents. After the Toney debacle, however, his handlers have shrewdly shielded him from heavy hitters, seeking opponents against whom his outstanding reflexes and considerable defensive skills have proved sufficient for victory.
While Nunn has moved up, Ashley is slimming down to what he insists is his natural weight for this contest and should consequently enjoy important physical advantages. He is also a genuine two-fisted puncher whose withering right hands and equally effective left hooks have accounted for 11 of his 18 victims inside three rounds. None, it must be said, have remotely approached Nunn's pedigree.
Ashley's progression from club fighter to world title aspirant has been achieved largely through his own efforts, unrefined talent combining effectively with unwavering determination. The record books reveal three losses in his first 12 outings, hardly the formline associated with a potential champion in the making.
'In the early days, I used to get kicked round a lot,' he said. 'I would go to the gym the day before a fight to be told it had been called off. Then there were times I'd not train for a few weeks, visit the gym and be told to fight in two days. The worst of it is that you want to say no, but you can't because there's a gas bill to pay. I've always been the opponent: there's hardly ever been a home-town fight, no one doing me any favours.'
After drifting through a succession of managers, Ashley joined Barney Eastwood's gym in Belfast in early 1991, a decision which marked a turning point in his career. In March, he was fighting for the vacant European light-heavyweight crown against the unbeaten Graciano Rocchigiani in Dusseldorf in what seemed at the time another piece of injudicious matchmaking.
The powerful German, a former world super-middleweight champion, was an overwhelming favourite yet, despite only having had a week's notice, Ashley looked unfortunate to be on the wrong end of a split decision. When a rematch was ordered, Rocchigiani promptly announced his retirement.
'I couldn't believe I'd been in with an undefeated world champion and he hadn't extended me,' Ashley said. 'A lot of people changed their attitude to me after that. It was as if they were saying, 'Sure, we've heard him talk, but maybe this guy really is as good as he thinks he is.'
Ashley regards the experience as the turning point in his career: he has not lost since. Eastwood's gym, with its unique chemistry of European and Latin American disciplines, has developed his natural skills, but more significantly has hardened his psychological approach. He has now left the Eastwood camp, joining forces with Frank Warren. There was no falling-out, Ashley simply tiring of the sense of isolation he felt in Belfast.
'I'm mentally very strong now,' he said. 'I've got the right attitude for this game. I'm a little stubborn, nothing's going to stop me if I want something. Being in the Eastwood gym, I learned you have to be totally professional in your outlook, fully committed to what you're doing.'
It is impossible to fault Ashley's dedication: since the new year, he has spent just seven days with his wife at the family home in Leeds, a level of abstinence rarely witnessed since the days of Marvin Hagler.
'The guy's got tunnel vision,' his trainer, Bob Pagett, said. 'I've been around a few world title fighters and although they'll always say they fancy it, you can tell when someone talks without conviction. But not this bloke. He's convinced he's going to do it.'
'It means a lot to me to be able to say I'm the best,' Ashley said during the final preparations for the trip to Tennessee. 'The money is nice for the family and to make my life a little bit easier, but what motivates me is a burning desire to be the best at what I do.' Will that intensity be sufficient to confound the odds and confirm Ashley's place in the pantheon of those who have achieved the impossible? Common sense demands that it cannot be done, but Ashley is far from alone in pointing out that logic and the fight game rarely have much in common.