While Spain is a world away from Manchester on a dreary day, the fallout from Guardiola's combustible relationship with Zlatan Ibrahimovic, whom he managed for an ill-starred season at Barcelona, and Jose Mourinho, whom he opposed in 11 'El Clasicos', is impossible to ignore, particularly since all three descended on English football's second city.
Ibrahimovic's spell at the Nou Camp ended with him branding Guardiola a "spineless coward" for speaking to him only twice in six months.
By contrast, the Swede regularly showers Mourinho, his coach when at Inter Milan, with praise, labelling him a "manager who lights up a room where Guardiola draws the curtains," an unusually colourful metaphor in football's normally monochrome lexicon.
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For Guardiola, Mourinho proved a more persistent threat and the pair publicly clashed, swapping insults and barbed observations at every opportunity.
Many believe Mourinho was the real reason Guardiola suddenly quit Barcelona for a sabbatical in 2012, exhausted by the strain of seeing Real Madrid win the title under the Portuguese.
On the pitch, Guardiola normally emerged triumphant, losing only two of their 11 meetings in Spain, a record that extends to three in 16 in overall career meetings.
On Saturday both men take perfect Premier League starts into the fixture, with City sitting marginally above United in the table on goal difference after both sides won their first three games.
If the former friends ever get round to speaking to each other, which they have not done for years, Manchester's new managers might find they share much in common.
Both have spent heavily in the close season -- United 150 million pounds and City 174 million -- in refashioning their teams, a process that also involved ditching established internationals with Bastian Schweinsteiger sidelined at United and Joe Hart loaned out by City.
Few will argue with their methods, however, and some pundits are already predicting that whoever emerges as Manchester's top team will win the Premier League.
Guardiola's City side have adjusted well to both a new way of playing, with last season's two holding midfielders replaced by a more free-flowing style that has liberated playmakers Kevin De Bruyne and David Silva, and of training, with a stretching regime designed to cut down persistent injuries.
Mourinho's impact has been no less marked, with world-record signing Paul Pogba making the expected impact in midfield and Marouane Fellaini looking a player reborn. Last week the manager's late introduction of substitute Marcus Rashford was rewarded with the injury-time winner over Hull City.
Guardiola, the master tactician, has spent much of the week deciding who will replace the suspended Sergio Aguero, with Nigerian international Kelechi Iheanacho the most likely starter. How City cope without the Argentine, and how United exploit his absence, will be crucial.
Up to now both managers have been keen to play down their own history, scarcely referring to the shadowy force on the other side of Manchester.
"I play against myself, not the others," said Mourinho this week in an effort to end discussion of the topic that will not go away. Guardiola has been equally dismissive.
But on Saturday, as the pair stand side by side on the Old Trafford touchline, the realisation will dawn that some people from your past are impossible to avoid.
(Reporting by Neil Robinson; Editing by Toby Davis)
