Ireland's injury crisis has intensified after Paul O'Connell was ruled out of Saturday's clash with South Africa.
O'Connell will miss the Guinness Series opener against the Springboks at Aviva Stadium because of a back injury that had showed signs of clearing up.
"Paul returned to training at the start of this week and took a full part in the squad sessions on Monday and Tuesday," read a statement released by the Irish Rugby Football Union.
"However, he suffered a reaction and will not be fit for selection for this weekend's game. Paul will see a specialist in the coming days and an update will be issued in due course."
The loss of O'Connell is a bitter blow to Ireland, who had already seen Brian O'Driscoll, Sean O'Brien, Rob Kearney and Rory Best ruled out of all three autumn matches.
In addition, Stephen Ferris is struggling with an ankle injury and at best will feature only in the final game against Argentina.
The absence of O'Connell, Lions captain in South Africa three years ago, has left a leadership vacuum for the Springbok Test with O'Driscoll and Best already in the treatment room.
A new captain will be confirmed at Thursday's team announcement with either number eight Jamie Heaslip or fly-half Jonathan Sexton filling the role.
On Tuesday Ireland's management had revealed their expectation that O'Connell would recover from his back problem, but now they being forced to delve even deeper into their playing resources.