Joe Schmidt is facing a nervous few weeks over his scrum-half options with Luke McGrath exiting Leinster's win against Toulouse and John Cooney failing to start Ulster's meeting with Racing 92 on Saturday afternoon.
With Kieran Marmion currently sidelined, it leaves the Ireland head coach with one fit, established number nine and, as good as Conor Murray is, even he needs back-up. Not ideal for Schmidt who is expected to name his Six Nations squad next week.
McGrath went down in what looked to be considerable pain on the hour mark against Toulouse and was subsequently replaced by Jamison Gibson-Park. The issue, said to be a knee, will be a worry for Leinster as well given third-choice nine Nick McCarthy is already unavailable.
But Ireland's need will focus more minds right now.
Marmion has not played since November due to ankle surgery, although Connacht coach Andy Friend stated this week that he is confident the English-born Ireland international will be back on the park in time for the Six Nations.
Cooney, who has played four times for his country, had been expected to start for Ulster today in their European clash at home to Racing 92 on Saturday afternoon but the Dubliner was reported as having a back problem. Dave Shanahan started in his place.
As for Leinster, McGrath's enforced exit was the one disappointment on a day when they put a highly-rated Toulouse side to the sword despite missing a rake of top-class internationals. Win away to Wasps next week and they top the pool and ensure a home quarter-final.
“A lot of guys have been fighting hard to get into the team and been unlucky in certain selections,” said Cullen.
“It was a good step today for a lot of them but next weekend is another challenge. We'll turn our attention to Wasps very quickly. It's a slightly longer week and we still have a lot of work to do to win this pool. It's a tough place to go, they won't give us anything easily.”