Arriving in January from Chelsea for a club-record fee, he was going to kick-start the David Moyes era after a stalled start.
But he couldn't. Not on his own. And United finished in their lowest position since the start of the Premier League in 1992.
There's a new manager this season. But the same problems have remained. And now another supposed saviour has been signed in the shape of Angel Di Maria with the promise that he'll solve everything.
It was clear at Turf Moor on Saturday that it's wishful thinking.
Louis van Gaal has obvious problems in midfield and in defence. It's the reason why he's agreed to pay £13.8 million to sign Daley Blind, a player who can offer cover in both those positions, as reported by the Manchester Evening News.
United's squad is well-stocked with attacking players. It's down in no small part to the £96.8 million they've spent on Mata and Di Maria in the last eight months.
They are the two most expensive signings in the club's history. But after just one game together, there's doubt whether they can be accommodated in the same team.
When the team bus pulled up at Burnley's stadium on Saturday morning, Mata and Di Maria stepped off together. It was the most comfortable they looked in each other's company all afternoon.
The issue for Van Gaal is whether he can use Mata, Di Maria, Wayne Rooney and Robin van Persie in the same team.
Rooney is his captain. Van Persie is a personal favourite. Di Maria is his record buy. The odd one out is Mata.
All four started together against Burnley. But with nearly £150 million of attacking talent on the field, United's forward play looked forced and stunted.
Van Persie brought one good save from Tom Heaton in the first half. But other than that, it was Burnley who, for large periods of the game, looked more likely to score.
Four players who cost 30 times more than Burnley's entire team should be able to deliver more.
Rooney was poor. So was Mata. Van Persie is only just starting to look match-fit after missing pre-season. Di Maria will take time to adapt to the Premier League and his new team-mates.
Van Gaal, at least for now, is sticking with his 3-5-2. Against Burnley, he had Rooney and Van Persie paired up front, Mata and Di Maria pushed up behind them and Darren Fletcher protecting the back three.
But it didn't work. It's an issue Van Gaal will have to solve before the restart of the Premier League following the international break.
Whether the Dutchman sticks with three at the back or reverts to the 4-3-3 he was known for at Ajax, Barcelona and Bayern Munich, it is Mata who looks most vulnerable.
If Van Gaal wants to persist with Mata and Di Maria as two of a midfield three, it relies on a mobile, physical holding midfielder playing behind them.
Fletcher's comeback from a serious bowel condition is inspiring. But it's debatable whether he can be that player.
It might yet be Blind, although the fans and probably Van Gaal would prefer someone like Arturo Vidal or Kevin Strootman.
When United line up in their next game against Queens Park Rangers on September 14, Van Gaal will have a settled group for the first time. The transfer window will be closed, and he will have to make do with what he's got, at least until January.
It will be clearer by then whether Mata is still the central figure everyone assumed he would be when he arrived from Chelsea. But he faces an anxious wait.