Trailing for 50 minutes to Craig Gardner’s early goal, United were facing a third consecutive league defeat, something they have not experienced since 2001, until the young Belgian struck twice in the second half.
His telling contribution spared United manager David Moyes from further scrutiny, in the wake of consecutive defeats by Manchester City and West Bromwich Albion, and lifted the defending champions into the top half of the table.
Sunderland had opted for an extra man in midfield and that, and another change of central defensive partnership for United, proved decisive in the first goal.
Ondrej Celustka beat Januzaj in a 50-50 challenge just inside United’s half and when Emanuele Giaccherini crossed, Phil Jones hit his clearance at Nemanja Vidic, who miscontrolled.
Gardner, playing as the extra man in midfield thanks to Celustka’s inclusion at right-back, pounced, steering a good finish inside the far post to give the home side a fifth-minute lead.
Jones and Vidic were the fifth central defensive partnership that Moyes had tried in as many games.
Jones made a nervy start, caught in possession by Gardner in the ninth minute, only to make an excellent recovering slide tackle in the penalty area. Vidic looked unsettled throughout by Jozy Altidore’s strength.
Januzaj’s selection was the most eye-catching of the visitors’ four changes. The teenager, who also qualifies to represent Albania, had been spared the long mid-week trip to play Shakhtar Donetsk in Ukraine because he is not registered in the Champions League.
A natural number 10, he started on the left, with Nani on the right, but the fact that both players shot narrowly wide from inside the D within the first 20 minutes showed they were quite happy to come inside.
Januzaj spent more time infield than along the touchline, leaving Patrice Evra to provide the width.
Nani ought to have equalised when Evra’s deep cross cleared Valentin Roberge’s head, but he steered his volley wide.
Bottom-of-the-table Sunderland were the equal of the away side in the first half, though, and Giaccherini their best player.
David de Gea made a terrific right-handed save when the 5ft 4in (163cm) wide man found himself in a generous amount of space from Adam Johnson’s cross and the Italian also shot over in the 44th minute after Johnson spread panic in the United defence.
Keiren Westwood saved Rafael da Silva’s powerful shot well in the 41st minute, but it was a rare moment of first-half action for the Sunderland goalkeeper.
Januzaj blotted his copybook in the 48th minute when he flopped over the merest hint of an outstretched leg from John O’Shea. Instead of getting a penalty, he was shown a yellow card.
But he soon made it up to the away fans. Ten minutes into the second half, he played a one-two with Evra and finished calmly just inside the left-hand post. His goal gave the visitors confidence and seemed to drain it from their hosts.
Four minutes later, Westwood had to make a good save after Robin Van Persie got on the end of a cross from Evra.
When Nani crossed from the right with the outside of his boot, O’Shea’s clearing header was weak and Januzaj volleyed it expertly into the net. With 29 minutes left, the game had been transformed.
Rafael’s slightly weak left-footed shot and Van Persie’s tame finish when played through by Antonio Valencia apart, United offered little threat thereafter, but Januzaj had broken Sunderland’s spirit.