Ireland have been training in the Cardiff City Stadium as they prepare for the match with Wales, which will kick off a tournament made to give purpose to international friendlies.
But as it stands, Denmark, the third team in their group, have only called up uncapped players and even futsal players due to a dispute over commercial rights.
Republic of Ireland trained at Cardiff City Stadium on Wednesday ahead of game with Wales
Martin O’Neill (right) has called on UEFA to act as Danish dispute threatens group integrity
Martin O’Neill took training as usual on Wednesday as he and assistant Roy Keane made their final plans for the Wales clash.
But Denmark’s preparations for their opening game against Wales on Sunday are in turmoil following a row between the Danish Football Association and their players’ association.
It could lead to Denmark fielding a scratch side of third-tier and indoor futsal players in Aarhus, as they did for their friendly with Slovakia on Wednesday night.
It would them with a far more inexperienced side than usual, missing established performers like Christian Eriksen and Kasper Schmeichel.
Ireland players were good spirits as they trained on the pitch for UEFA Nations League game
Roy Keane (right) and coach Steve Guppy (left) watch on as they make final preparations
The Republic’s Ala Judge (front) and team-mates warm-up during training
Such an uneven contest for that game would put the Republic at a serious disadvantage in the group.
Ireland are not due to play Denmark until the next international break, during the middle of October – by which time the dispute could feasibly be resolved.
‘If it does develop this way there would certainly be a degree of unfairness about it,’ O’Neill said at his press conference.
‘I think everyone would accept that if Denmark play players from lower divisions against Wales then it certainly gives them a massive advantage.
‘I think UEFA probably should (act), and will as well, if it develops in the direction that it looks as if it is going. UEFA will have to make some sort of statement about it then.
‘It wouldn’t be fantastic news for the competition itself, but we have to see if things get resolved in the next couple of days and Denmark play their full-strength side.’
Roy Keane watches on as he hopes his side get off to a winning start in Nations League
Callum Robinson takes part in training for Ireland ahead of Thursday’s crunch clash
The Republic’s troubled build-up to the fixture suffered another blow when Preston midfielder Alan Browne was ruled out by a calf strain.
Browne joins James McClean, Robbie Brady, James McCarthy, Scott Hogan and Shane Long on the casualty list.
Bournemouth midfielder Harry Arter withdrew from the squad following a spat with assistant manager Roy Keane, while West Ham defender Declan Rice made himself unavailable as he considers switching allegiance to England.
O’Neill said: ‘Whatever goes on with some of the players at club level, whether they’re playing or not, they come in and get themselves prepared for the game.
‘Whatever we have we will just give everything we possibly can during the course of the match. The players have done that before my time and during it as well.’
Alan Judge (left) trains ahead of the tournament which could provide Euro 2020 qualification
Martin O’Neill has called on UEFA to act with Denmark possibly handing Wales advantage
O’Neill will be taking charge of his 50th game as Republic manager in Cardiff and believes the new Nations League is a step forward for European football.
‘The competition has a bearing on the European Championships later on,’ he said.
‘Despite the fact that there seems to be a feeling that no-one totally understands what it’s about, I think we’ve got the gist of it.
‘If you pare it down this competition has taken the place of a play-off situation for some third-placed teams, so I think it is really important.’