It is almost half a century since a player was allowed to move directly between Manchester United and Liverpool, and when the Argentinian defender Gabriel Heinze thought he might break the tradition in the summer of 2007 he was swiftly disillusioned. A request to move up the M62 to Merseyside after falling out with Sir Alex Ferguson was dismissed, and instead he ended up with a not-unwelcome transfer to Real Madrid.
Any suggestion that such a switch between the North-west's two great clubs would be easier now that Liverpool are in a trough gave Ferguson a perfect exit line at his media conference on Friday when the topic was offered up for consideration. "Some question, that. What do you think?" he almost snorted before making his way off to training with a broad smile.
Until then United's manager, 71 a fortnight ago, had been in fine form in reflecting on the transformation in fortunes of the two clubs since he arrived at Old Trafford in the autumn of 1986, with United in the relegation zone and their greatest rivals having won eight of the previous 11 League titles.
"I knew the challenge when I came here," he said. "At one of my early press conferences we said we couldn't afford to go 25 years without winning the title. It was 26! They had fantastic teams, they had a good management structure, two great managers in a row.
"Bill Shankly created it and Bob Paisley carried it on. It was a tragedy for Bill to quit and then they go and win three European titles with Bob. Shankly deserved one of those. That period of great teams, great players, they were the dominant team, no question."
If Arsenal, Chelsea and now Manchester City have in turn become the real threat to United's annual ambition of winning the Premier League, the rivalry with Liverpool endures as intensely as ever. Ferguson traces it back to socio-economic roots and the building of the Manchester Ship Canal in the 1890s, taking trade inland from Merseyside.
"From that day on the rivalry was big. It's not just football, geographically there is fantastic competition between the two clubs. Of course, being the two most successful, that has intensified over the years and that won't change. If we were bottom of the League or in the second division it would be the same." Having finished a remarkable 37 points ahead of Liverpool last season, United already lead them by 21 this time. Yet the gap in recent matches has rarely been more than a single goal. In September at Anfield, Robin van Persie's late penalty won it, against the balance of play; in three bitter meetings last season United drew away in the League with another late goal and lost 2-1 in the FA Cup before winning by the same score at Old Trafford, when Ferguson raged that Luis Suarez "should never play for them again" after refusing to shake Patrice Evra's hand.
The September meeting, coming a few days after the Hillsborough Independent Panel had handed down their damning judgment, also had a tense build-up, with both captains appealing publicly to supporters to desist from inflammatory chants. In the end, balloons were released, hands shaken and the headlines were monopolised by the referee, Mark Halsey.
This time Ferguson was even prepared to admit to a grudging admiration for Suarez: "I read one report saying they might play Suarez wide right, Brendan Rodgers was quoted. I hope he does. I think he should play right-back!"
There was acknowledgement, too, for Steven Gerrard's extraordinary record of not having missed a minute in the League this season. "Him and [Graeme] Souness were probably the best midfield players they've had." This being Liverpool, however, there are of course limits, which were apparent when he was asked if it was a shame Gerrard had never won the League. "I don't think it's a pity. You're joking."
Liverpool v Manchester United is on Sky Sports 1 today, kick-off 1.30pm