Not only has Sir Alex Ferguson left his position after 26 years, chief executive David Gill has stood aside as well 11 years after succeeding Peter Kenyon.
Yet there was no power vacuum. For Arnold and executive vice-chairman Ed Woodward have divided up Gill's responsibilities between them.
And a day after Ferguson's exit was confirmed, which itself came on the morning following the first rumours of an impending departure, David Moyes was installed as his fellow Scot's successor.
"To go through the succession we have been through represents a big change," Arnold told Press Association Sport. "The way the transition was achieved characterises how well the club has been run by David and Sir Alex."
Shirt sponsors Aon, experts in risk management, were asked for their input into what, in Ferguson's case at any rate, was one of the riskiest decisions United have had to make for a quarter of a century.
In the end, it was decided to go with the Scot's own belief Moyes was best suited to the role.
"The man widely accredited as having the best judgement in football had a long period of time to think about the inheritance he was passing on," said Arnold.
"He wanted to make sure the squad was in the right shape and the timing was done from a position of strength. That is exactly where the club is.
"When you look at the preparation that went into that and the position Sir Alex put himself in to make that transition, as an example of how to prepare, even down to how we managed the communication around it, it was an exemplary job."
If there is an unknown about Moyes in terms of operating at the very highest level, his credentials as a manager are solid. Less certain is how Woodward and Arnold will fare in the often bewildering, shark-infested world that is football recruitment, which seems to have different rules to any other sphere of society.
Yet Arnold claims Woodward, whose job it now is to negotiate transfers and new contracts, is not quite the rookie everyone imagines him to be.
"David (Gill) had been preparing both myself and Ed in quite some detail about the way it all works," said Arnold.
"Ed has been particularly well briefed about the operations of how we transact with players, both the fantastic ones we already have and if there is an opportunity to buy. As befits a man of his nature, David did a fantastic job in the nurturing he has given us, the preparation and handover.
"Laying the flight path to the runway is all about preparation. Everyone is well aware of the change. But from the inside, in the context of having to make the change, it has been fantastically well done."
- David Moyes
- Sir Alex Ferguson