The appointment of Luiz Felipe Scolari as Chelsea manager has sent tremors through the football world. Even at Manchester United, where such developments are usually dismissed with a shrug of the shoulders, the aftershock threatens to be felt long after the conflict over Cristiano Ronaldo's future has been resolved.

Consider the following: Scolari and Sir Alex Ferguson do not get on, the legacy of a climate of distrust in club-versus-country disputes over the past five years; Scolari and Carlos Queiroz, the United assistant manager, despise each other; Scolari has publicly and privately advised Ronaldo to take the “opportunity of a lifetime” (one that will, incidentally, arise every year until he takes it) and join Real Madrid; Scolari endorsed and facilitated Ronaldo's announcement last week about his plan to move to Madrid; Scolari and Ferguson will be in direct and, in all probability, volatile competition next season, with Chelsea and United battling at the top of the Barclays Premier League.

Even before Scolari's appointment at Chelsea was announced on Wednesday evening, there was a sense of unease at Old Trafford at the Brazilian's role in greasing the wheels of a bandwagon that Ronaldo maintains will drive him to Madrid. Ronaldo has spent the past 2 weeks in the care of Chelsea's manager-in-waiting - and, given Portugal's fine start to the European Championship finals, could remain so for another 16 days - and that will fuel conspiracy theories at a time when Ferguson and Queiroz are struggling to get their messages across to a player whose head has been well and truly turned.

The expected fireworks never quite materialised between José Mourinho, the former Chelsea manager, and Ferguson, with the two men mostly showing each other a reverence and deference that was quite at odds with their contrasting backgrounds and their conflicting philosophies on football, politics and human nature. Some viewed it as a superficial friendship born out of convenience on both sides; Arsène Wenger, the Arsenal manager, was said to regard it as an unholy alliance against a common enemy.

With Scolari, though, it promises to be quite different. Sources at Old Trafford say that Ferguson and Queiroz have found it “impossible” to work with Scolari during his time as Portugal coach, when they have frequently sought assurances from him over his use of Ronaldo.

The issue first arose in November 2006, when, against United's wishes, Scolari played the forward in a European Championship qualifying match against Kazakhstan and he aggravated an ankle injury. Queiroz reacted by saying that: “Scolari behaves like one of those people you lend your car to when it has a full tank of gas and they use it all week, crash it and then leave it on a side street with no gas and aren't even polite enough to tell you where they left it.”

There is, to be blunt, no love lost between Scolari and Queiroz, which would make it all the more intriguing if, as suspected, the United assistant manager is high on the Portuguese Football Federation's list of candidates to take over from the Brazilian. When Queiroz attended one of Portugal's qualifying matches for Euro 2008 - ostensibly to monitor the form of Ronaldo and to scout Nani, who subsequently joined United from Sporting Lisbon, and Miguel Veloso, who may do so in the future - Scolari claimed that the Mozambique-born coach was “here to prepare his application for the job of Portugal coach in the future”.

So far, so childish, but next season the rivalry will be real, heightened by the intensity of the title race. Throw in a long-time Ferguson adversary in Wenger, whom it is hard to imagine seeing eye to eye with the belligerent Scolari, and an increasingly truculent Rafael Benítez at Liverpool, and it is easy to imagine that next season's title race will be the most acrimonious yet. Mourinho, who has resurfaced in the relative tranquillity of Inter Milan, would have loved it.