The January signing has made a dream start to his career with the Bhoys and, without the 24-year-old's strike, Ronny Deila's men wouldn't have been anywhere against Inter.
But, while Armstrong stole the headlines with his finishing in the final third, another January signing also did the business for Celtic, impressing onlookers just as much as the man who moved to Celtic Park with him and netted against the Italians.
Another young winger who has flourished since joining Celtic, Mackay-Steven took more touches than Armstrong, completed more of his passes than his team-mate and won an impressive four dribbles against the Italians.
The 22-year-old also drew three fouls and hit the target with two of his three shots.
Armstrong outshone Mackay-Steven in the crossing department but the point stands that both are looking good when it comes to assessing the Bhoys' January signings, even if one name will stand out above the other in the match reports that will go down in history due to the goal he scored.
Deila rates the pair highly and it's easy to see why. Does a big Celtic future lie in wait for the young duo?