Assistant coach Simon Easterby described the move as “precautionary” after the back row tweaked a hamstring. With Thursday pencilled in as a down day, O’Brien has basically been afforded 48 hours to regain full fitness before the weekend’s duties.

“A tight hamstring,” said Easterby. “Nothing unusual there. He might have had it in the past. Other players have these things and we have to just manage him. He doesn’t have to train again until Friday.

“We are pretty comfortable with the amount of work he has done in the early part of this week and last week to feel that we could manage his tightness.”

Thankfully, there are no other new injury concerns.

Garry Ringrose was called in to cover for the absence of Luke Fitzgerald – ruled out for the entire tournament on Tuesday with a medial knee ligament injury – but the young Leinster player will not stay with the squad for the duration.

Joe Schmidt had already spoken of the need for Ringrose to continue his development with his province where he is still only in the foothills of his senior career. His presence in national camp owed to the fact that Leinster have no game this weekend.

Schmidt also voiced his doubts over Ringrose’s ability to withstand the physical stakes against a Welsh team that contains two enormous centres in Jamie Roberts and Jonathan Davies but there are no such concerns over Stuart McCloskey.

At 6’ 4” and over 17 stone, the Ulster centre has the beef to cope.

“Stuart has played against some physical teams this year,” said Easterby. “Toulouse weren’t a small team if anyone was up in Ravenhill. I was and they did pretty well. He has gone well in the physicality stakes against Saracens as well.

“He is a big man and so are Wales (sic). They have threats across their back line and they are the same size as Stuart. He is well able to step up. That wouldn’t be an issue for him if he was to be selected.”

Warren Gatland pulled a few surprises on Wednesday when naming his side for the Ireland encounter. Most interesting is the decision to start Justin Tipuric at openside flanker instead of Dan Lydiate with captain Sam Warburton moving to blindside.

So, not just one breakdown jackal for Ireland to counter but two.

“They had a few issues over Dan Lydiate in terms of fitness status so they know with Warburton and Tipuric in the back row that they have two guys who can cover the ground well,” said Easterby.

“They are going to be a threat at breakdown time. We know that is a big part of the game.

“They have used those two in the World Cup pretty effectively as well so, no, I’m not surprised. What it shows now is that they have that depth and if they pick one over the other then it doesn’t change the quality of what they have to put out on the park.”