Despite having scored just one Premier League goal in 2016, overall the Nigerian has impressed during his debut season in the top flight by finding the net 14 times for the Hornets.
Such form has captured the attention of the Red Devils, who have set aside £20m to secure the 26-year-old's services, according to The People.
The report adds that United officials plan to push ahead with the move, regardless of whether manager Louis van Gaal keeps hold of his job at Old Trafford.
However, it is believed that the Red Devils will face competition for Ighalo's signature from rivals Arsenal.
The frontman recently spurned a number of chances as United recorded a 1-0 victory over Watford.