Robin van Persie opened the scoring early on after decent work from Mata allowed Ashley Young to pick out Antonio Valencia at the far post. David Marshall saved well from the Ecuadorian, but Robin van Persie followed up the rebound with a dreadful header right at the Scot. Fortunately the ball bounced kindly and he was able to put the second rebound away to put United into an early lead.
Cardiff were looking outclassed, and United nearly made it 2-0 through Van Persie again, who fired across goal after he and Ryan Giggs got in each others way at the near post following a corner. Yet Cardiff would grow into the game, with Craig Noone's driving runs allowing them to break into the United half and threaten on several occasions.
After the second half, United were no better and Cardiff improved further, with Noone again the danger as nerves began to set in around Old Trafford. But after the visitors had spurned a couple of chances, Juan Mata bagged a very cheap debut assist, rolling the ball to Ashley Young, who darted past two defenders and arrowed a shot into the top corner from outside the area.
That pretty much killed the game off, and the rest was a humdrum affair with both sides knowing the result had been set. Mata was unable to contribute too much more before being withdrawn for Adnan Januzaj, who went the closest to bagging a third by taking Patrice Evra's cutback in his stride but drilling just wide of Marshall's far post.
So, a fairly easy three points, the return of two key players and the addition of potentially our best. Elsewhere, Arsenal drew away to Southampton so United are now 12 points off the top, while Everton were hammered by Liverpool and lost Romelu Lukaku to injury, strengthening one rival for the top four and weakening another. Not a bad day's work.