The 17-year-old is named in Erik ten Hag's midfield for the quarter-final encounter at Old Trafford, which kicks off at 20:00 GMT.
It is a great reward for the youngster's progress after he scored in the friendly against Cadiz, during the winter training camp.
Mainoo, born in Stockport, becomes the second Academy prospect promoted into senior football for the first time by Ten Hag, after striker Charlie McNeill came off the bench during the Europa League defeat to Real Sociedad.
He is the eighth man to debut under the Dutchman, with new arrival Jack Butland waiting for his bow, as Tom Heaton makes his first start between the sticks.