Ospreys and Wales centre Gavin Henson has been ruled out for four to six weeks because of a broken hand.
Henson is said to have suffered the injury in the win at Harlequins
He is likely to miss the rest of the group stages of the Heineken Cup, but not the national team's Six Nations opener against England on 2 February.
Henson was injured during his club's EDF Energy Cup win at Harlequins.
"From reviewing the match video, it appears that an opposition boot accidentally came into contact with his hand," said team doctor Rhydian Lewis.
"Gavin was able to complete the game, albeit in some pain, and he was checked out immediately after the final whistle by our medical team, who diagnosed a suspected fracture of the fourth metacarpal in his left hand.
"We then secured the earliest available appointment for him to undergo an x-ray on Monday, when our initial fears were confirmed.
"Gavin has now had his left hand set in a lightweight cast and, while he will be able to continue with his fitness training, it is expected that he will be unavailable for selection for a period of four to six weeks while the bone heals."
News of the injury came only two days after Henson issued a public apology after he and a group of friends were reported for rowdy behaviour on the train home from the Harlequins game, which Ospreys won 19-8, and spoken to by transport police on arrival in Cardiff.
It is not the first time the 25-year-old has been involved in controversy.
Gavin is an excellent rugby professional and no-one works harder at their skills and conditioning than he does
Ospreys director of rugby Kevin Hopkins
After starring in Wales's 2005 Grand Slam and impressing on the Lions tour to New Zealand, his career was blighted by a series of injuries and arguments with team-mates over comments in his book.
He missed out on World Cup selection due to his lack of fitness, but impressed in Wales's 34-12 loss to South Africa last month - only his second Test start in a year - and has been picked out as an important player for the future by new national coach Warren Gatland.
Ospreys director of rugby Kevin Hopkins described Henson as an "excellent rugby professional", saying: "No-one works harder at their skills and conditioning than he does."
He added: "Both Gavin and our medical staff have worked hard in an effort to get him on the pitch against Ulster on Friday.
"But in the end we have decided that Gavin's long-term wellbeing is best served by him getting the hand properly sorted over the next few weeks.