Ireland head coach Joe Schmidt admitted that losing his captain, lock Paul O'Connell, ahead of Sunday's quarter-final against Argentina is a huge setback.
O'Connell was forced off the field shortly before half-time in Ireland's Pool D win over France on Sunday with a serious hamstring injury.
The injury forced the 115-Test veteran into premature Test retirement.
“That’s going to leave a big hole,” said Schmidt.
“He obviously calls the lineouts. He dictates a lot of the play along with Johnny (Sexton) and we’re going to have to manage around that.
“He’s obviously a leader by nature. Not having his voice, but also his incredible example around [is a loss].
“It’s incredibly disappointing for Paul as it is for us because we feel we need his leadership and his playing talent and at the same time he would have loved to have finished in a knock-out match in a World Cup.
“To be honest, it’s a quiet presence that takes up a lot of space. His influence on others doesn’t have to be stated – it’s very much demonstrated by what he does.
"Because he does it, other people follow and what he does is commit himself to every facet of preparation.
“He’s a good lad as well, he likes a bit of craic in the environment.
"He tends to pair himself up with the guys who skive off a little bit and therefore it probably allows him to relax a little bit and at the same time it keeps them on track.”