The former Scotland international was in Leeds on Saturday evening to watch boss Steven Gerrard play for a Liverpool legends team.
He was waiting on a taxi with his wife Hayley when the attack is said to have occurred.
McAllister was punched in the face and required ten stitches. Though the 54-year-old refused to comment, he did say: “It’s in the hands of the police.”
A source close to the Scot told to the Scottish Sun: “Gary was in hospital from about 3am to 12 noon. His face is a mess. He must have at least 10-12 stitches in his top lip.
“It was totally unprovoked by just one bloke. He came from nowhere and just hit him simply because Gary is a former footballer.
“It beggars belief. Gary’s pretty shaken up by it all. He and Hayley have always tried to keep out of the limelight.”