For years, Arsene Wenger's Premier League feuds centred around Sir Alex Ferguson. And then Jose Mourinho turned up.
Ever since the Portuguese manager first took over at Chelsea in 2004, the pair have had a number of public spats - most famously their touchline scuffle in October 2014 just a few months after Mourinho's 'specialist in failure' press conference.
However, according to a fairly stunning revelation in a new book from author and self-proclaimed Mourinho confidant Rob Beasley, things were even more brutal away from the TV cameras. That's after Beasley revealed a threat the then Chelsea manager allegedly made about Wenger two years ago.
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The incident Beasley describes apparently came a few days after Mourinho's now infamous February 2014 press conference in which he stuck the knife in on Wenger's inability to win trophies.
Those comments were themselves spurred on by the Arsenal manager's own thinly veiled claim that Mourinho was plotting against him by selling Juan Mata to Manchester United.
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You might think the public dressing down of his rival's trophy credentials would have been enough to get things of Mourinho's chest. However, as Beasley recounts in an extract serialised by the Daily Mail today, he went even further - and was far more violent - with his comments in private just a few days later.
"A few days later he was still pumped up about it all, telling me: 'When Mr Wenger criticises CFC and Man United over the deal with Mata...I will find him one day outside a football pitch and I will break his face."
Even for Mourinho that is pretty extreme and, fortunately for Mr. Wenger, he hasn't made good on his alleged promise to date.
Perhaps the fact that, one month on from his apparent threat, Chelsea ran out 6-0 winners against Wenger's Arsenal was enough to satisfy his urges.
Nevertheless, the story shows just how much each manager makes the other's blood boil, even after almost a decade of competing against each other.
Football fans will have to wait a little while longer before we get to see if tensions have cooled between the pair now that Mourinho is at Manchester United.
With Arsenal not set to play the Red Devils until November 19, there is plenty of time for cutting comments and subtle digs in press conferences to come. We're unlikely to hear either manager publicly threaten to break the other's face any time soon, however.