Ruan Pienaar will miss Ulster's upcoming trip to Scarlets this Friday night.
The South African sustained an injury to his right knee against Leinster last week in the Guinness Pro 12.
The scrum half will see a specialist this week to ascertain the extent of the problem.
In more bad news for Ulster, both Rob Herring and Rodney Ah You are out for up to six weeks with knee ligament injuries.
Ulster’s Ruan Pienaar in action against Leinster ©INPHO/Billy Stickland
Roger Wilson is also ruled out of the Scarlets match with a knee injury.
Remaining on the sidelines are Ireland internationals Stuart Olding and Jared Payne due to a calf and kidney problem respectively.
Payne is a doubt for the Six Nations campaign as a result of his injury.