Nigel Worthington has voiced his disappointment after Shane Duffy opted to play for Republic of Ireland.
The highly-rated Everton defender, 18, had been selected in the Northern Ireland squad for next Wednesday's friendly against Albania.
Worthington had been looking forward to working with the youngster who had represented Northern Ireland at various age groups.
However, it emerged on Monday that Duffy had switched his allegiance to Republic of Ireland, following in the footsteps of players like Darron Gibson and Marc Wilson who were also born in Northern Ireland.
Worthington believes there could be flaws in a system that allows players who have come through the age groups with one nation to represent another country at senior level.
"There is a loophole where a player can walk away after a lot of time and investment has gone in," he said: "Until it is resolved, that's where we are.
"Shane has represented Northern Ireland all the way through from a very early age to the Under 21s.
"I am a big admirer of Shane. I like him as a player and a lot of work has gone in with different coaches. I am disappointed with the situation as he is going to be a very good player."
Worthington added: "Shane has now decided, after discussing the matter with his family, to represent the Republic. As a manager, I have got to respect that."