April’s delayed final would be the chance of a first trophy for both Mourinho and his former club since their acrimonious parting two years ago.
The smallest of English football’s major prizes would be one of unusually high stakes for the two managers involved.
For Mourinho, the chance of redemption and evidence that his powers remain after a damaging end to his Old Trafford reign.
For Ole Gunner Solskjaer, a potential springboard to a new trophy-winning era at United and a much-needed pointer to his own capabilities of returning them to former glories.
Solskjaer needs to prove he can get over the line – or, at the very least, to a final.
This is his fourth semi-final appearance in the space of a year after limping out of the previous three.
He goes into it with United in their healthiest position at this stage of a season since Sir Alex Ferguson’s retirement. Level on points with Liverpool at the top of the Premier League, this is the first time in eight seasons United have reached the turn of the year with genuine title ambitions.
That includes Mourinho’s time, when the now Tottenham manager recorded their highest finish post-Ferguson – second in 2018.
Should United overcome Manchester City, it would strengthen the belief that they are realistic contenders for Liverpool’s crown. Defeat, and fresh doubts would rise again.
This is the team that capitulated in the Champions League group stages and were humiliated by Mourinho in a 6-1 defeat at home in October.
They look a very different prospect after a 10-game unbeaten run in the league, which would see them move clear at the top if they win their game in hand.
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Mourinho would relish the opportunity to make a statement against Guardiola, too, but nothing would be more delicious than coming back to haunt United.
In this most unpredictable of seasons, their resurgence under Solskjaer has a sense of fragility to it with numerous come-from-behind wins, late goals and penalties. That is not to undermine the run they are on or the strides they have made over the past 12 months, which are undeniable.
But that league form has been punctuated by alarming lows in defeats to Istanbul Basaksehir and RB Leipzig in Europe and the first-half performances against West Ham and Southampton.
If they are negatives, the positives can be found in the resilience shown by United and a winning habit at a time when it still feels like they are yet to hit their stride.
They still need a top-quality centre-back to partner Harry Maguire and improve a defence that has kept just four clean sheets in 15 games.
They need a consistent centre-forward to lead the attack, while there is an obvious over-reliance on Bruno Fernandes.
His importance to their ambitions cannot be overstated – and it would be unfair to cite it as weakness. As if Liverpool haven’t been dependent on Virgil van Dijk, City on Kevin De Bruyne or Tottenham on Harry Kane.
It is coming up to a year since Fernandes’ protracted move from Sporting Lisbon was completed.
Last January when United faced City in the first leg of their Carabao Cup semi-final at Old Trafford, Ferguson was said to have cut a chastened figure at half-time after witnessing Guardiola’s team race to a 3-0 lead.
United had already lost to Arsenal on New Year’s Day and would suffer further defeats to Liverpool and Burnley before Fernandes’ arrival.
Solskjaer’s days looked numbered until Fernandes proved the inspiration behind an unbeaten run in the league that took them to the end of the season and third place.
Is that evidence of how close United were to coming together that it took just one player to affect such a transformation?
Or is it proof of just how precarious their resurgence is that their fortunes lie in the hands of one man?
That is what is so fascinating about this campaign that has already witnessed wild fluctuations among all of the leading clubs.
City, for instance, conceded five against Leicester and also lost to Spurs – but will move above Liverpool if they win their games in hand.
Chelsea were top at the start of December and currently sit ninth.
Champions Liverpool put seven past Crystal Palace before Christmas and haven’t won a game since.
Add to that unpredictability the fact that United are extremely streaky under Solskjaer and it would be foolish to greet their current position with anything more than cautious optimism.
But victory tonight would be further evidence that this is not just another false dawn.