“I am disappointed that he has ruled himself out of the two games but our focus is on the players who are with us and I will address this situation after the international period.”

Lafferty started Northern Ireland’s Nations League opener against Bosnia – a 2-1 home defeat – to earn his 68th cap, and the 31-year-old is one of O’Neill’s most experienced players.

Northern Ireland face Austria in Vienna on Friday and Bosnia in Sarajevo on Monday, needing to pick up points on the road after losing their opening match 2-1 at home to Bosnia.

Lafferty’s absence could present an early opportunity for Rotherham striker Kyle Vassell, who has been called into O’Neill’s squad for the first time at the age of 25.

The Milton Keynes-born forward qualifies through his mother and is the latest player to be brought into the Northern Ireland team through the efforts of head scout Andy Cousins to widen the pool of talent.

Peterborough boss Grant McCann was the one to alert Cousins to his eligibility two years ago, and Vassell has been kept in touch since.

“I met the gaffer (O’Neill) earlier in the year at Blackpool and he said I wasn’t far away,” Vassell said. “This time around I’ve been fortunate enough to make the squad, so I’m delighted.”

Vassell describes himself as “strong, powerful and quick enough” as well as “very determined”. While he prefers to play up front, he knows in O’Neill’s system he may well find himself in a wide role – something he has also been doing with Rotherham this season.

“I’m definitely a striker,” he said. “Can I play wide? Yes. Will I play wide? Yes. And I’ll still give everything I’ve got. I just want to play. It’s a cliche that I don’t care where I play but that’s truly honest of me. As long as I play, I’m happy.”

Earning a first international cap would be the next step on a remarkable journey for Vassell, who played for a string of non-league clubs after being released as a youngster by Brentford, moving into the professional game with Peterborough in 2013.

“It’s all a bit surreal, to be fair, because I had an extensive non-league CV, I had quite a few clubs, and I found it quite difficult to get out of non-league,” he said.

“Even though I’m in the Championship now, I still see myself as a non-league footballer, still that same person. The fact that I’m now, almost, an international, is a bit surreal, but it’s something I’m embracing and enjoying – and hopefully I can play for Northern Ireland as well as I have been for Rotherham.”

Vassell admitted he had only visited Northern Ireland a handful of times – “it’s something I definitely should do more” – and if he can establish himself in O’Neill’s squad that will quickly take care of itself.

“I want to do the best I can in a Northern Ireland shirt to make my mum and my family proud,” he said.

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