The England Under-21 international had been expected to complete his switch to the Bluebirds before tonight's 11pm transfer deadline.
Despite interest from numerous Premier League clubs, it has been confirmed by Cardiff that Zaha will play for them until the end of the season, according to Sky Sports News.
Manager Ole Gunnar Solskjaer is quoted by The Mirror as saying today: "He has pace, power, technical ability and goes past people. He will give us that extra change of tempo in the final third, where we really need it. He will bring that little final bit to find pockets of space and produce a bit of magic."
The 21-year-old has struggled to make a name for himself under United manager David Moyes since moving to Old Trafford from Crystal Palace last summer, having made just two league appearances this season.