The Belgian midfielder is unlikely to have a place in the first-team next season after a largely disappointing season with the relegated Black Cats and the Red Devils ready to sanction a permanent move for their once highly rated prospect.
Citing sources, ESPN reports that United are willing to offload the player if they receive bids between £8m ($10.3m) and £12m from interested clubs. Januzaj will enter the final year of his contract at the end of the month and the Red Devils will be keen to cash in on him rather than allow him to leave on a free transfer next summer.
The 22-year-old was touted as one for the future when he made his breakthrough at United during the 2013/14 campaign under David Moyes. He made 27 appearances in the Premier League under the former Everton manager but failed to continue his progress under his successor Louis van Gaal in the following season.
After struggling for game time during the Dutch manager's first campaign, he was sent out on loan to Borussia Dortmund at the start of the 2015/16 campaign, but returned midway owing to a lack of opportunities. He spent the rest of the campaign playing with the reserves.
Mourinho's arrival did not change his fortunes as he was again sent on loan to Sunderland where he made 28 appearances during the recently concluded campaign. The Portuguese manager revealed earlier in the year that a decision regarding Januzaj's future was already made, but refused to divulge further details.
The Belgium international is unlikely to be short of suitors, with Telegraph reporting in April that Paris Saint-Germain, Lyon, Monaco and Marseille were keeping tabs on the midfielder, while also including Serie A clubs Roma, Napoli, Juventus and the two Milan clubs.