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.
We caught up with Millar to reflect on his Anfield experience and why he feels he has come back to Liverpool a better player following his time in Scotland…
On Monday's impressive 6-0 victory...
The first half was a bit harder because we had the wind blowing against us and we started off a bit slower because of that, but in the second half we came out flying and got the goals. Even when teams do sit off against us, which they do most weeks, we can play around them and still get the goals. We've just got to keep doing what we are doing and keep it going.
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On Jones' quick-fire hat-trick…
It was incredible. I literally thought the game was over but somehow, someway he goes and scores three goals! Fair play to him and he is in great form. It's really good to play alongside him.
On scoring goals and getting assists from the left flank…
Ever since I went out on loan, playing on the left wing has become my position nowadays. I think I learned a lot while I was out on loan and it's helped me to come back here and do well. That's important for me and I've got to keep going, keep getting goals and keep getting assists.
On making his first-team debut against Shrewsbury…
Honestly, that night was unbelievable. I couldn't explain it. Obviously, I've played at Celtic Park and at Ibrox, which was unbelievable, but I've never had that home feeling with the whole crowd supporting us. It was unbelievable, I couldn't believe it.
On making his family proud in Canada…
It was probably the proudest moment of my life. To be the first Canadian to play for Liverpool is one of those things that's hard to really take it in. It was overwhelming.