The hardest thing for Manchester United when it comes to encounters with Liverpool these days is that they bring them face to face with everything they could have had.
Like everybody else across Europe, United were aware of Jurgen Klopp's star rising through the stratosphere of German football a decade ago.
Like everybody else, they wondered if, one day, he would prove a good fit for them.
When they face Liverpool, Manchester United will be reminded of what they could have had
An emotive figure, Jurgen Klopp has transformed Liverpool's fortunes since becoming boss
That culminated last June when he guided Liverpool to their sixth European Cup
He would have, too, but the fact is that Liverpool's subsequent sell to Klopp was better both in substance and timing than United's had been, proof perhaps that they had stolen a march on their great rivals even before the evidence started to appear on the actual field.
Liverpool's current status as English champions-elect hardens with each passing week and victory. United? Well, the very best we can say about Ole Gunnar Solskjaer's team is that it can be occasionally effective.
Solskjaer and United's committed travelling support must hope Sunday turns out to be one of those better days. United are, strange as it sounds, the only team to have taken a point from Liverpool in the Premier League this season - courtesy of a 1-1 draw last October - and these days such occasional, fleeting returns to relevance are all that English football's fallen titans have to play for.
If that sounds rather stark then it should. United, for all Solskjaer's talk of small progress, continue to drift while Liverpool surge forwards. Klopp suggested on Friday that Liverpool cannot hope to dominate over a period of years like United did under Sir Alex Ferguson or indeed Liverpool prior to their last title win in 1990.
The popular German is on course to lead Liverpool to the Premier League title this season too
United considered appointing Klopp but Liverpool's sell was better in substance and timing
For all Ole Gunnar Solskjaer's talk of small progress, United are continuing to drift
The quality of the Premier League is just too high, was the argument. Yet United never started a season with 20 wins from 21 games. Nobody did. Liverpool should win this season's edition by an enormous margin.
If Klopp really isn't thinking 'monopoly' as he looks in to the future then other people at Anfield will be. 'To be honest, I don't think it is possible to get to United's level of dominance ever again,' said Klopp.
'We have made a few good decisions and that is why we have got better. And our players are getting better - that is why we are coming up. But it is now more likely that the other clubs will compete for different things because of the financial power of a lot of teams.
'I wouldn't say United won a lot of games 4-0 or 5-0 but they were predicted wins, and that is clear. 'We won a lot of games 1-0 or 2-1 and had to really dig in properly to do so, so I don't see this type of thing.
'But it is important