Still, they figured it was undeniable that he would make them better. And, crucially, that he could turn a horrid season into a decent one.
Fourth place in the Premier League was six points away, not an insurmountable gap. They had a return leg semi-final at Old Trafford coming up against Sunderland in the League Cup and they faced Olympiacos, a more than manageable opponent, in the round of 16 of the Champions League, meaning that you could hope for a run (and some nice revenue) in Europe.
Six weeks on, things haven't improved. Sunderland, of course, knocked them out on penalty kicks, 48 hours before the Mata deal was clinched. Fourth place is now 11 points away. And, most of all, Olympiacos weren't the cream puff opponent many expected, winning 2-0 at home on Tuesday night.
The outlook was bleak, it is now bleaker. In fact, it's fair to say that almost any United manager other than David Moyes (or his predecessor) may well have been let go at this stage. Instead, we're seeing much of the punditocracy close ranks around Moyes. They peddle this line that he inherited an awful squad in need of a full revamp from Sir Alex Ferguson and that, while he may have made the odd mistake, it's primarily the players "letting him down".
Probably for two reasons. First, many backed Moyes to do well, in part because of his record, in part because he was hand-picked by Sir Alex. Nobody wants to admit they were wrong until they have no other option. I should know, I was among those who thought he was the right choice.
Second, Moyes is not just a fully paid-up member of the football establishment, he's also a pleasant, decent man. He doesn't have many enemies and he has plenty of personal relationships, of the kind some of his colleagues do not have. It's the old "do unto others" thing and, yes, it does matter when it comes to getting the benefit of the doubt.
It can't be the fantasy that there's some serious problem with this group of players. True, some are on the slide. But this is the same bunch that won the Premier League by 11 points last season, minus Paul Scholes' 771-minute swansong, plus Adnan Januzaj, Wilfried Zaha (before Moyes decided to banish him), Marouane Fellaini and, for five games, Mata.
The "rubbish squad" trope only works if you buy into the myth that Sir Alex had such magical powers that only he could be successful with this bunch. And, frankly, that's absurd.
Moyes has been criticised for many things, but perhaps the most disappointing is the failure to develop the many younger players on this team. Phil Jones, Chris Smalling, Rafael, Tom Cleverley, Shinji Kagawa, Danny Welbeck, "Chicharito" Hernandez... all of them are 24 or younger. You would have expected at least a couple of them to kick on to the point where they could become bona fide starters which, you'd imagine, is what Sir Alex had in mind for them. Instead, they've all either been treading water or regressing. And that has to be, in part, down to the manager and his coaching.
It still makes sense to believe in Moyes as the right man for United, but it does get more and more difficult every week.
It's the question Sunderland owner Ellis Short doesn't want to answer. Not in public, at least. Would you swap today's League Cup final - and a chance to win your first trophy since 1973 - in exchange for a guarantee that you would not be getting relegated this season?
To the bean counters, it's obvious. Survival keeps the revenue streams intact; cup runs don't mean much. Birmingham City won the League Cup in 2010-11, beating Arsenal in the final. And yet they were relegated, failed to make a return to the top flight and now find themselves in a pool of red ink and 16th in the Championship. Surely that can't be a good thing?
To the romantics, it's a no-brainer. This is what you yearn for as a supporter. Not so much winning, as being 90 minutes away from winning. It's that hope, that anticipation, that sense that you have every time you kick-off, except this time you have the kind of achievement which, locally, you'll be talking about for years.
And to some degree, that's the disconnect of the modern era. When seven of the Premier League's top eight have been the same every season for the past five years (and will be the same this year) for many the indelible memory is the cup final, not the number of years you successfully avoid relegation.
Sunderland's opponents today, Manchester City, know the feeling. After all, they were Sunderland until Sheikh Mansour rocked up and shifted the paradigm. A League Cup win for City would be nice, of course it would, as would the "day out" at Wembley. But they are also aware that the goalposts have moved, that after winning an FA Cup and a Premier League crown in the last three years, another League Cup would feel almost routine, not quite the type of achievement that is passed on from parent to child and on to the next generation.
It's a bit like the cracks about Arsene Wenger winning nothing but the imaginary trophy you get for finishing in the top four.
Qualifying for the Champions League is a bigger achievement than winning a cup because it requires consistent performance over 38 games and is less influenced by luck, whether good or bad. Plus, it's far more important in terms of revenues, both direct (TV money and Champions League money) and indirect (brand-building, season ticket sales, sponsorship deals).
Your head tells you this is what matters. In the same way the brains of Sunderland supporters - and Short, who ultimately pays the bill - know that staying up matters more than a trophy. But your heart knows otherwise. Your heart knows that you are a football fan for reasons that are intangible. You will continue to be a fan, regardless of whether you are relegated and revenues fall and you're forced to sell your star players.
But what will stand out will be the emotions. And few emotions in English football, for a supporter outside the top eight, can match the minutes before a Wembley final kicks off.