When the two sides met in the Carling Cup semi-final early last month, Derby had only just appointed Nigel Clough, with the team spiralling towards consecutive relegation. As the new manager watched from the stands, the outstanding Kris Commons scored a superb goal while a young Derby side outplayed the champions in a way that no other side has been able to this season.
Today, United return to the scene of that defeat and Derby, rejuvenated by Clough, are in form and hungry to inflict what would be only United's second defeat since Nov 8.
'We're more confident this time round,' Commons said. 'Before it was just a dream to try and beat them. Now we know it is half-dream, half-reality.'
Some 15 years ago Clough staged his own heroics against United, scoring twice as Liverpool came back from 3-0 down to draw 3-3 at Anfield. Commons was only 10 years old at the time but as a Nottingham Forest fan he knew all about the family Clough. Forest had been relegated the previous season, Brian Clough's last campaign before retiring.
Commons returned to the City Ground as a player in 2004, playing there for four years before moving to Derby in the summer. He could have been excused for thinking the all-pervasive influence of Brian Clough would be at least diluted by moving to Derby County. Six months later and Clough's son Nigel is his manager.
'You don't play for Forest for that long and not appreciate how important Brian Clough is to the club,' Commons said. 'Everywhere you turn people are talking about the European Cup, about the history.
'That's why Nigel's been so brave in doing what he has done: he knows that by being a manager everyone is going to expect him to be just like his dad. It was a big decision after 10 years with Burton.
'He does have an aura about him. I've never worked with a manager like him – he's brilliant. He's just so incredibly calm, makes things simple for you.
'The really incredible thing is that he still plays with us in training and he is such a good player he could still get in the team. We just love coming in to training now, are desperate to play the games. We're walking round with our chests out and our heads held high – it wasn't like that earlier in the season.'
With three straight wins – including coming from two down to beat rivals Forest in the previous round, Commons scoring the winner – Derby are in fine fettle and know that United will not underestimate them a second time.
'The first time we played them, considering where we were in the league, we were all set up for disaster,' Commons said. 'But all the chaos surrounding the club took every ounce of pressure off us. It was great to play and score against the very best.'
Derby went down courageously at Old Trafford in the second leg, losing 4-2. That was the first time a team had scored twice against them since they lost to Arsenal in November.
Even if Alex Ferguson starts with Edwin van der Sar – 'the bogeyman', Commons jokes – in goal, United will not appear indomitable to him and his colleagues.
They know they have got what it takes.
FA Cup fifth roundDerby County v Manchester UnitedKick-off: 4.30pmTV: ITV1
Team news: Wayne Rooney is not quite ready to return from his hamstring injury but Patrice Evra and Jonny Evans are fit. With Fulham coming to Old Trafford on Wednesday, Ferguson will use the strength of his squad. Derby's Wales defender Lewin Nyatanga is suspended.
Key clash: Fletcher v Savage. Derby ruffled United in the Carling Cup by being physically dominant in midfield. Darren Fletcher will try to give United the edge, but Robbie Savage is bound to be at his confrontational best.
Derby County (4-4-1-1): Bywater; Connolly, Albrechtsen, Addison, McEveley; Barazite, Green, Savage, Teale; Commons; Hulse.Manchester United (4-4-2): Foster; Rafael, Vidic, Evans, O'Shea; Park, Scholes, Fletcher, Nani; Tevez, Welbeck.