Callum O'Dowda is out of the Republic of Ireland's Nations League clash with Wales in Dublin on Tuesday evening.
The Bristol City midfielder suffered dizziness following a collision during the first half of Saturday's 0-0 draw with Denmark and sat out training on Monday morning as a result.
Manager Martin O'Neill said: "He is out of the game tomorrow night, precautionary as much as anything else.
"He wasn't feeling great at half-time, but he can't remember when it actually happened during the course of the game - we are trying to find out ourselves - so the best thing to do was to take him off, take him out of it and then any sort of reaction, it would be too early to consider him for tomorrow night."
Wales will be without star players Aaron Ramsey, who has withdrawn from the squad for "family reasons", and the injured Gareth Bale. The Real Madrid star has returned to Spain for treatment on muscle fatigue.