The Canada international returned from a loan spell with Kilmarnock in January and was handed a starting berth by Neil Critchley for the fourth-round replay at Anfield.
It was a senior debut - and an unforgettably proud moment - for the 20-year-old, who arrived at the Academy in 2016.
Since that night, Millar has been a regular starter for the Reds' U23s and netted three goals.
Most recently, he was on the mark and provided a brilliant assist for Curtis Jones to complete a hat-trick during Monday's 6-0 win over Sunderland in the Premier League Cup.