The Red Devils announced the news on Wednesday.
Sporting (h/t Sport Witness) provided the financial details of the transfer, with United set to pay a €55 million (£46.5 million) initial fee and a further €25 million (£21.1 million) in potential bonuses. The Portuguese side are also entitled to 10 per cent of the profits of a future sale.
Per the announcement, Fernandes still has to pass a medical and agree to personal terms with the Red Devils.
The 25-year-old will become United's first signing of the January window once he agrees to personal terms and passes his medical. His transfer has been one of the biggest storylines of the window, and speculation was rampant during the summer as well.
The midfielder was one of Europe's most productive players in his position last season, scoring 20 goals and adding 13 assists in the Primeira Liga. He has bagged eight goals and seven assists so far this season, carrying Sporting's attack on his own at times. The team sit 19 points behind rivals Benfica in the Portuguese league standings.
Per Bleacher Report's Sam Tighe, his heavy usage in Sporting's system makes him a difficult player to project. Every attack runs through Fernandes, leading to a ton of production. He's unlikely to put up similar numbers in the more difficult Premier League, where his role will be smaller.
His ability to create and score will be a welcome addition to a United side that has struggled in both departments. With 36 goals scored in the Premier League, they sit well behind the top three of Liverpool, Manchester City and Leicester, who have all scored 52 or more.
The January transfer window will shut at 11 p.m. GMT on Friday, so United have a few days left to conclude this deal.