It was meant to be Robert Lewandowski’s game.
The Bayern Munich striker is Poland's captain and their undisputed star. He scored 13 times in qualifying for Euro 2016, and plundered 42 goals in all competitions for his club in 2015-16.
Northern Ireland boss Michael O'Neill lauded Lewandowski before the match, and fielded a five-man defence and two holding midfielders in a nod to the qualities of Poland's talisman.
The Ajax striker, 22, is regarded as one of the most exciting young strikers around and has been heavily linked with a move to Serie A.
Milik could have opened the scoring after just five minutes, but his weaker right foot let him down. He never stopped trying, though, and caused the Northern Ireland defence all kinds of trouble.
Milik thus became only the fourth Polish player ever to score in the Euros, helping them to their first-ever victory after six winless games in 2008 and 2012.
If Sunday's performance is anything to go by, Milik seems destined for bigger things and a move away from the Eredivisie looks inevitable ahead of next season. Ajax can only hope he can replicate this form in Poland's remaining games in order to boost his price tag.
Coach Adam Nawalka, meanwhile, will take encouragement from Poland's historic victory, but is likely to be slightly troubled by Lewandowski's underwhelming showing.
It has been a long and intense season for the Bayern striker, who made over 50 appearances in all competitions in 2015-16, and he may be feeling a little jaded.
Perhaps the prospect of pitting himself against many of his club-mates will bring the best out in Poland's key man.
