The highly rated 16-year-old made a name for himself at the 2013 Northern Ireland Milk Cup where he scored a very impressive 10 goals in five games for Watford's academy side, and finished the tournament as leading scorer.
Since then Obi has caught the eye of Arsenal and Liverpool according to a report in The Daily Mail, but it seems United have trumped them both by making the first move on the youngster.
The Nigerian-born player will get the chance to show what he is capable of at United's state-of-the-art Carrington training complex this week, where he will train alongside the club's other young academy stars.
The Championship side appear desperate to retain the services of Obi, and have offered young striker a place on their scholarship scheme in addition to a one-year professional playing contract.
Obi will find likely find it difficult to turn down the lure of playing in a prestigious academy like that of United's, but considering the differing levels of the two clubs he might find chances of breaking through to the first team easier by staying at the Hornets.
United's new search for football's next big star came to light yesterday when it emerged that they had offered trials to two young Portuguese talents from Benfica.
Joao Filipe, 15, has been dubbed the next Cristiano Ronaldo, and Joao Virginia, also 15, is a goalkeeper.
To see Obi in action check out his Milk Cup goals in the video below, he is wearing the number 9 shirt for Watford.