He will also sit out the province’s visit to Castres tomorrow evening.

Ireland’s opening Six Nations test is just two weekends away, against Scotland in Murrayfield. Cullen described O'Brien's issue as a minor, grade one injury - a mild strain or tear - although the matter has taken longer to right itself than expected.

“I thought he would have been available this week,” said Cullen on Thursday morning. “It’s been a bit slower than we would have thought. I would have thought he would have turned it around for this week.

“There is always a chance with guys coming back, there is always a chance for complications with injuries even if they are very, very minor.

“I would imagine he would be fine for the start of the Six Nations. He wasn’t a million miles off for this week but he’s not good enough to go this week."

Cullen will also have to do without lock Hayden Triggs in France this time after the Kiwi was hit with a three-week suspension for making contact with the eye area of Australian scrum-half Nic White during the meeting with Montpellier last Friday.

Triggs has returned to the team with great effect in recent weeks.

“Hayden has been going, really, really well. He’s put his hands up and said that he shouldn’t have pushed Nic White in the face basically.

“There was no intent on his part but players realise that you can’t do that. It was an unfortunate incident, he has been through his hearing (on Wednesday) and we move on now.”