Ulster have been hit with another injury blow just as they go into a crucial period in the Guinness PRO12 and European Champions Cup.
South African tighthead prop, Wiehahn Herbst, is expected to be out for at least two weeks after suffering a calf issue during training on Tuesday.
He will miss the top of the table PRO12 games against Leinster in Dublin tomorrow and Scarlets next Friday (January 6) in Llanelli.
Ulster hope that he will be available for the fifth round game in the European Champions Cup away to Exeter on Sunday, January 15.
Meanwhile, Andrew Trimble is expected to start against Leinster on Saturday in the Guinness PRO12 derby clash at the RDS, Dublin (3pm).
He had made good progress last week after suffering an injury with Ireland during the November Test series, but the game against Connacht came too soon.
Trimble is expected to start against Leinster with Peter Brown also likely to be available.
Rob Herring will lead Ulster into the game in Dublin, where they have only won once against Leinster since 1999.
Director of rugby, Les Kiss, said he spoken with both co-captains Herring and Trimble this week.
“Andrew is just coming back for his first game so we felt it was right to give Rob the captaincy this weekend.”