Ian Baraclough feels that Mark Sykes took away opportunities from another youngster that actually wanted to play for Northern Ireland.
The Oxford United midfielder is in the process of switching allegiances to the Republic of Ireland.
Sykes has played for Northern Ireland at both U19 and U21 levels, making 11 appearances at the latter age grade.
The 23-year old had worked under Baraclough when he was in charge of their U21 side.
In announcing his squad for Northern Ireland's upcoming Nations League games with Romania and Norway, Baraclough admitted he was shocked to hear of Sykes' decision.
"I was disappointed to hear from Mark and what he had to say," Baraclough explained.
"[I] spoke to him again over the weekend just to make sure that it was the right thing for him to do and that he was totally sure.
"We've had to move on now. It's not been an easy weekend - for him, and his development with the Irish FA - and the way we've nurtured him as a player coming through the age ranks.
"But the rules are there, and he's spoken to me about it, and he wishes to play for the Republic."
Sykes looks set to follow in the footsteps of the likes of James McClean and Shane Duffy - both of whom played U21 football for Northern Ireland before switching to the Republic.
All players born in Northern Ireland are eligible to also play for the Republic of Ireland. This is despite a challenge from the IFA at the Court of Arbitration for Sport a decade ago.
Former Sligo boss Baraclough admits to not being happy with the rules as they stand, but concedes a young player could make such a decision "at any time".
"I'm bitterly disappointed for coaches, managers, the people that have been involved in bringing someone like Mark Sykes through the system," Baraclough explained.
"He played for me at 21's, but that was just a small part of his development.
"There's many coaches and managers that have spent time, the IFA's finances on someone like him.
"It's disappointing to get to a stage where he's actually been in three squads and he's decided to move to the Republic.
"For me, I want players that are fully focused, determined to play one hundred per cent for Northern Ireland and so we have to move on.
"It gives somebody else a chance now, but unfortunately I can't unwind - for a young player that would've been developing the same time as Mark - what chances he would have had maybe coming into a squad.
"That's the disappointing thing. We spend time, we spend finances. All the people that have had a hand in Mark's career so far - it's disappointing on their behalf really."
Sykes joined Oxford from Glenavon in January of 2019. Last season, he played 39 times in all competitions for the U's.
He also scored in their League One playoff final defeat to Wycombe Wanderers at Wembley in July.
Baraclough feels there was a future ahead of Sykes with the North, "He's not the finished article. I think he's still got a good way to go, and he would say that.
"I felt he was on the right route to going and being possibly a strong player for Northern Ireland, [with] the chance of amassing a good amount of caps.
"That's no longer the chance, but it does give someone else a chance.
"For someone like Joel Cooper to come into the squad, it's a strange twist of fate that he's an Oxford United player also.
"But someone that we were speaking about as an association and a group of staff for maybe March's get-together against Bosnia.
"He's had a really good campaign for Linfield, he's taken that now into his early career with Oxford.
"He's hit the ground running. Speaking to coaching staff at Oxford, they're delighted with him and he's got a really good future."
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