The 22-year-old Irishman, who has signed an 18-month deal at Bramall Lane after joining from Irish League side St Patrick's, stepped off the substitutes' bench for the last 15 minutes in the 3-1 third-round defeat to Aston Villa.
McAllister said: "I was delighted to get on the pitch and get my debut out of the way. The gaffer told me to play my own game, get in the box and see if I could make myself a hero.
"When the draw was made I thought I might be included, depending on injuries and suspensions, but it is a dream come true really."
The Blades have also signed fellow Irishman Seamus Conneely, 22, a central defender from Galway United, for an undisclosed fee.