Aston Villa's ascent to the Premiership summit since Gregory arrived from Adams Park in February has surely been beyond the wildest dreams of the chairman, Doug Ellis. Tuesday's Uefa Cup win in Spain was their 19th in Gregory's 25 games, with only three defeats.
But encouragingly for the posse attempting to erode Villa's four-point advantage, Leicester have an outstanding record in the clubs' Premiership meetings. In six matches, which have tended to have a bitter edge because of what the East Midlanders' perceived as the poaching of Brian Little, Villa have not managed a single victory.
Gregory, who was on the coaching staff at Filbert Street himself under Little, will warn his players against the O'Neill factor. For even if the Leicester manager looked less than elated by the conclusion of his part in the Leeds saga, the players and supporters clearly regarded it as a massive psychological boost.
The contest promises to be close. Despite their position, Villa have under-achieved in terms of goalscoring while Leicester's matches tend to be drawn or settled by the odd goal. Stan Collymore, commended for his work-rate after the games at West Ham and Celta Vigo, urgently needs to score, having failed to do so in the League since Gregory's first match.
Collymore's tussle with another shaven giant, Matt Elliott, and the duel between Gareth Southgate's calm authority and the muscular power of Emile Heskey may go some way towards determining the outcome. Heskey and Tony Cottee should be warned that no Englishman has scored against Villa since Bolton's Bob Taylor in April.
Stranger still, all four goals against Villa this season have been by Scandinavians: Mikkel Beck, Trond Soltvedt and two Stromsgodset players. If all else fails, O'Neill might be advised to throw on Sweden's Pontus Kaamark.
Villa's closest pursuers, Manchester United, are also in the area, facing a Derby side who were themselves second before losing three in a row to what their manager, Jim Smith, described as "amateur goals". United, who drew at Pride Park last autumn after trailing 2-0, return buoyed by the flowering of two spinal partnerships.
Dwight Yorke and Andy Cole have between them accounted for half of the 14 goals scored by Alex Ferguson's side in their last three games. And Gary Neville and Jaap Stam could be United's best centre-back combination since Steve Bruce and Gary Pallister, with the added bonus of Wes Brown's emergence at right-back.
Brown, 19, has been eased in gradually by Ferguson, avoiding the fatigue that seems to be afflicting Michael Owen and damaging Liverpool's title aspirations. Having scored only twice in his previous 10 appearances for club and country, Owen was relegated to the bench against Valencia.
For Nottingham Forest, without a win at Anfield since before the Clough era, the best hope of extending Liverpool's run of three points from 15 may lie in defending deep in order to deny Owen and Robbie Fowler space to run into. Further frustration could force Liverpool to reassess the job-sharing arrangement between Roy Evans and Gerard Houllier.
Two clubs who have already changed managers this season, Tottenham and Newcastle, meet at White Hart Lane where the reception for George Graham will be revealing, especially if Ruud Gullit's team play with the style cherished by many Spurs fans.
Graham is too hard-nosed to believe in omens. However, his first match with Leeds, at Coventry, had the same pattern as Spurs' defeat at Leicester - 1-0 up, 2-1 down, to reverse a slogan from his days as the Arsenal anti- christ. Spookily, he then lost at Elland Road... to Newcastle.
Exasperation at Leeds is subsiding into embarrassment as the search for a successor drags on. Although the players showed spirit in Rome, the subsequent rebuff by O'Neill and the unexpected delay over David O'Leary are hardly conducive to turning round a mediocre run against a Chelsea side unbeaten since the opening day.
Also tomorrow, Arsenal go to Blackburn, where they routed Rovers 4-1 en route to the championship at Easter. While the home manager, Roy Hodgson, has declared defiantly that "winners don't quit and quitters don't win", he may need the Gunners to reprise their habit of under-estimating plain opponents (eg Charlton, Leicester, Sheffield Wednesday and Southampton) if he is to avoid being a loser again.