Iain Henderson, one of Ireland's best players at this year's Rugby World Cup, is expected to be unavailable for the 2016 Six Nations after Ulster revealed he suffered a major injury in last week's PRO12 clash with Edinburgh.
Henderson's performances this year had indicated that he was ready to step up and fill the void left by retiring captain Paul O'Connell in the Ireland second row.
The 23-year-old lock, who can also feature at flanker, will be sidelined due to a "significant hamstring tear" according to a statement from Ulster on Monday, with no timeframe put in place for his return.
The Irish Times however report that Henderson is likely to be ruled out for six months with an injury similar to the one O'Connell suffered in the World Cup pool match against France.
Henderson has won 23 caps for Ireland, with eight starts, since his debut against South Africa in 2012.
Ulster said: "Iain (Henderson) suffered a significant hamstring tear (left leg). He will be reviewed by a surgeon and his ongoing management and prognosis will be clearer after this consultation has taken place."