Leinster have confirmed Seán O’Brien has had a setback in his rehabilitation process and is in a race against time to feature in the upcoming Six Nations.
The Ireland international missed the recent Guinness Pro14 wins over Munster, Connacht and Ulster with a tightness in the hip, but it had been hoped he would return in time for the final two pool games of the Champions Cup.
The 30-year-old however has been ruled out for both those games and Leinster 's latest updates suggests the powerful flanker is now a doubt for Ireland’s opening game away to France on 3 February.
"Seán O'Brien saw a specialist this week for the tightness in his hip as it hadn't responded to treatment as we would have liked," the province confirmed.
"As a result he had a small procedure and the target is to have O'Brien back available during the 6 Nations window."
Cullen admits O'Brien will be missed within the group, but wouldn't be drawn on the backrower's potential availability to Joe Schmidt and Ireland.
"He's a great leader in the team, Seán," he said.
"He's being managed relatively conservatively and had a small procedure. Hopefully he won't be out too long."
Asked when O'Brien might be back before the end of the Six Nations, Cullen remained non-committal.
"It's hard exactly to put a timeline on it. We'll see how he comes through (operation). He shouldn't be a million miles away."
The Tullow man has played just 276 minutes over four club games this season, but said last week that "there shouldn’t be any concerns" over his participation in the Six Nations.
O’Brien’s last outing was against Exeter Chiefs at the Aviva Stadium last month.
Leinster have also said there is no timeline for Adam Byrne’s return to action. The winger has not featured since injuring his knee on Ireland duty in the Autumn Series and will undergo surgery today.
"He was not responding favourably to the conservative approach and as a result saw a specialist this week," Leinster said.