Leinster second-row Quinn Roux is expected to be out for five months after dislocating his shoulder in last weekend's PRO12 win over Glasgow.
The South African forward only recently recovered from a broken collarbone which makes this a disappointing setback for coach Joe Schmidt.
"I don't know if you saw it but he was starting to get a little bit of rhythm," said Schmidt.
"He is a very strong man and we were starting to get a little excited about what we thought he could bring to help us out.
"It's really devastating for him. He is really gutted. Just the way he fell, his shoulder dislocation, he is probably looking at 16 to 20 weeks' rehabilitation."
Leinster have received some positive news on the injury front however as Rob Kearney and Brian O'Driscoll are expected back in January while Luke Fitzgerald is progressing well.
"Luke's ahead of schedule," said Schmidt.
"We got a bit excited that he might be a bit further ahead than that but unfortunately we have to be patient because he is a top-quality member of our environment."