Ighalo, the first Nigerian to play for United, sow flashes of brilliance during the five minutes he featured and came close to scoring shortly after coming in for strike Anthony Martial on the dot of 90 minutes.
Three minutes after being thrown into the action, Ighalo was located by Fred inside the Chelsea vital area and he toe-poked one straight at goalkeeper Willy Caballero, who saved with his knees.
The visitors had earlier taken an unassailable lead via killer headers by Martial in the 45th minute and Maguire in 66 minutes.
It was a sad night for Chelsea as their two goals were cancelled by VAR.
For the visitors, the victory takes their point haul to 38 and sees them on the seventh position on the log and closing in on top four finishing to guarantee Champions League qualification.