Moyes had to know that when taking the job.
Ferguson, who officially retired following the season-ending 5-5 draw against West Bromwich Albion, leaves behind arguably the greatest legacy in English soccer history. Manchester United's Premier League crown was his 13th in the top division, and his 20 English league titles are a near-untouchable record.
Walking away at 71 years old, it's fully possible that he's leaving a couple more titles on the table. Ferguson is the most beloved figure in Manchester United's club history, a human embodiment of consistent excellence through multiple eras. It was Ferguson who helped build this club into a worldwide monolith—the type that every manager aspires to hold.
And it's also Ferguson who personally recommended Moyes to be his replacement. The legendary manager pushed hard for Moyes to take over at Old Trafford, which has places an even greater honor on this offer.
The situation also jolts roughly 15 million volts of pressure into an already high-profile position.
Being chosen by Alex Ferguson to take over Manchester United is akin to being called into Darth Vader's office and being given controls of the Death Star. Or taking over the Boston Celtics in the post-Red Auerbach era. Or something really, really awesome but at the same time first-time-you-watched-a-scary-movie-as-a-child frightening.
Ferguson even knows that Moyes will face an adjustment period. He warned his replacement of the "enormity" of running Manchester United at a presser last week.
"I think the enormity of the club – he will soon realise that anyways," said Ferguson (per The Guardian). "The global brand and number of sponsors we have here, he has to fit into that. I don't think that's an issue, though, and the most important thing is the team."
Moyes spent 11 years at Everton prior to taking the United job, gaining the respect of Ferguson and everyone at Old Trafford in the process. He knows how high-profile a job Manchester United is. Moyes isn't some alien with no knowledge of English soccer dropped down and told to manage the world's most famous club.
And yet even though he signed a six-year deal to manage the club, it's fair to wonder whether Moyes will be able to handle everything that goes with his job. Ferguson noted the global brand and sponsors the club has—something the Glazers have emphasized heavily since taking over.
That's all without even mentioning the current roster composition.
Still a little over a month from his tenure officially beginning—Moyes doesn't take over from Ferguson officially until July 1—he will already be faced with a vexing decision that could alter his entire tenure.
Wayne Rooney, the 27-year-old forward whose name has become synonymous with Manchester United for young fans, has reportedly asked for a transfer. This was the second time in three years transfer talks have come up with Rooney, and there have been fears the relationship is beyond repair.
Ferguson benched (arguably) his best player for one of United's most important matches of the season: the second leg of their Champions League matchup with Real Madrid. While of course that doesn't automatically mean Rooney is on the outs, it leaves Moyes with a mess that needs immediate cleaning.
Moyes and Rooney have a relationship going back to their days at Everton, and the forward has denied Ferguson's claims that he officially requested a transfer. There was a "clearing of the air," according to the Mirror. Whatever that means.
But it's the Rooney situation, along with many other interesting roster quandaries, that Ferguson leaves for his successor.
There is no doubt the Red Devils have a full cupboard of elite talent, as last summer's spending splurge leaves a Premier League-winning roster spearheaded by Robin van Persie. But NBA coach Pat Riley's "Disease of Me" theory isn't just something that exists stateside. It goes across the pond, and Moyes will find out quickly that navigating egos on an elite roster is often just as difficult as being a genius tactician.
That's what the enormity Ferguson speaks of means. There are sponsors to contend with, head-case players and arguably the most passionate fanbase in the world all staring Moyes in the face and expecting greatness.
They of course can't expect a repeat of the Ferguson longevity—that was a fluke even in Fergie's era.
What they will look for is a continuation of the excellence. The yearly contention for a Premier League crown, keen eye for developing young talent and all-knowing confidence even in the face of adversity. In today's win-now-or-get-the-hell-out culture, the expectations at places like United are already stifling without the historical connotations.
All you need to do is look at Jose Mourinho's career to know nothing lasts forever. Mourinho is arguably the world's greatest manager at the moment, bringing forth enormous success everywhere he's gone. And yet the 50-year-old has skippered six teams since 2000 and has never lasted longer than three years anywhere he's been.
Moyes and Mourinho are obviously two different men. But Ferguson's looming presence and the defense of a Premier League championship are enough to make any manager have a steep adjustment period.
The question is whether anyone will gives Moyes time to figure it all out.
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