The 18-year-old joins from English Premier League side Watford.
According to the latest report from The Athletic, Onuachu is the subject of major interest from the Premier League
Xisco Munoz’s side made sure of a top-two spot with a 1-0 home victory over Millwall, thanks to Ismaila Sarr’s 11th-minute penalty.
In an interview with Alternative Sport, the boxer from Watford expressed some interest in working with WWE in the near future.
The deal is worth €17 million.
Red Devils skipper, Harry Maguire, Anthony Martial, Marcus Rashford and Nemanja Matic were preferred substitutes ahead of Ighalo in the tie
The Red Devils, who won the last of their 12 FA Cups in 2016, suffered a 2-0 defeat to Manchester City in the semi-finals of the EFL Cup on Wednesday, while Watford will enter the contest off the back of a 2-1 defeat to Swansea City in the Championship on January 2.
United face Ighalo’s former club Watford in the FA Cup.
Both clubs played in the Premier League last season.
After two seasons at Serie A outfit, Udinese, the 27-year-old joined Vladimir Ivic’s side on a five-year deal for an undisclosed amount.
The 27-year-old centre-back was born in the Netherlands, where he began his career with Groningen.
The 43-year-old has agreed a one-year contract, with the option of a further year held by the club.
The managerless Hornets are in the bottom three heading into the final round of fixtures, behind 17th-placed Aston Villa on goal difference, while Arsenal are 10th and have one eye on next weekend’s FA Cup final.
Aston Villa climbed up to 17th in the standings after Tuesday’s 1-0 win over Arsenal and are above the drop zone only because of a better goal difference than Watford in 18th
Watford are 17th in the table, three points above the relegation zone, following Friday night’s 3-1 defeat to fellow strugglers West Ham.
The Cherries are three points adrift of safety before fellow relegation rivals Watford and West Ham United lock horns on Friday, but Southampton are comfortably in mid-table with two games remaining
Ighalo was crushed when Watford lost 2-1 to Crystal Palace, missing out on the chance to face Manchester United in the final.
All their goals came before halftime of the match expected to be a nervy contest between the sides perched immediately above the drop zone
The game began with a minute’s silence to mark the passing of former Newcastle and Ireland manager Jack Charlton, who won the World Cup with England as a player, after he died on Friday aged 85
Buoyed by a much-needed win last time out, Watford will be looking to climb clear of the Premier League relegation zone when they host Newcastle United at lunchtime.