Ireland attack coach Mike Catt says the team still do not know the extent of Dan Sheehan's injury, but admits not having the hooker for the World Cup would be a "big loss".

Catt said he was seeing a specialist in Dublin and that an update would be provided later this week.

"He's one of the best hookers in the world currently," said Catt.

"He's going to be missed in whatever team he plays in, but we have full faith in Rob Herring and Ronan Kelleher and Tom Stewart obviously, who's showing his face at the moment."

Leinster's Sheehan, who was outstanding in Ireland's Grand Slam triumph earlier this year, was replaced by Rob Herring during Ireland's 29-10 win over England at Aviva Stadium.

Sheehan's injury, which the Irish Independent reported could see the 24-year-old miss Ireland's entire pool stage campaign in France, is another concern given that Leinster team-mate Kelleher has not played at Test level since the Six Nations because of a hamstring issue.

However, with uncapped Munster hooker Diarmuid Barron joining Ulster duo Herring and Stewart, and with head coach Andy Farrell saying Kelleher will "definitely" be fit for the World Cup, Catt says the Samoa game is an opportunity for the other players to impress.

"It gives these guys an opportunity to have a pop his weekend. Depending on the diagnosis, we'll see how we get on in the back-end of it," he added.

"He [Sheehan]'s a world-class rugby player, he's been fundamental in terms of how we play our game so he'd be a big loss to anybody."

'We've got to make a decision somewhere along the line'

Farrell will announce his 33-man World Cup on Monday and Catt indicated that there has been spirited debate among the coaching staff regarding selection calls.

"Yeah, there's always good conversations to have and that's what you want when it comes to a good group of players like that," he said.

"Listen, I've been on the receiving end of it as well in my career so it's not pleasant for certain people but you've got to make decisions somewhere along the line.

"With the way the game has gone and your HIAs [head injury assessments] and yellow cards and all that sort of stuff, any one of the guys that we release currently that have been with us for eight weeks, nine weeks, we'll see them back. That's just the nature of the game at the moment.

"So, they're still massively part of the game and the squad, but we've got to make a decision somewhere along the line," he added.

Ireland, the world's number one ranked team, beat Italy before facing England, but Catt says they must be "more clinical" when they take on Samoa in Bayonne in their final warm-up game on Saturday.

"We need to play our way," he said.

"We know what's coming with Samoa. They're a big, physical side and we need to make sure we're playing our game and playing at a tempo that we've trained at the last eight or nine weeks to play at.

"It's making sure that the guys are in the right frame of mind. It's their final opportunity so it's about putting down a gauntlet for selection.

"We want to go into the World Cup on a massive high. We haven't performed particularly well against Italy or England, we were rusty or clunky as they put it, so it's an opportunity for us now to fix the fixables."