Paddy McNair is set to stay in hospital in Helsinki for a third night as concern grows over the abdominal injury he sustained in Northern Ireland's draw with Finland, sources have told ESPN FC.
McNair looks set to become the latest Manchester United defender sidelined after he was forced off in Sunday's World Cup qualifier.
The 20-year-old suffered bad bruising to the ribs and the area immediately below.
Irish Football Association doctor David White has stayed with him and he is in close contact with United.
United are already without left-back Luke Shaw, who is out for six months with a broken leg, and they are waiting to see if Marcos Rojo is fit to return after a hamstring injury when they face Everton on Saturday.
Manager Louis van Gaal ended the 3-0 defeat to Arsenal with wingers Ashley Young and Antonio Valencia occupying the full-back positions.
McNair started at right-back as Northern Ireland beat Greece 3-1 to qualify for Euro 2016 and then drew 1-1 with Finland.
The converted midfielder, who signed a new contract in February, has made one appearance for United this season.