Gloucester have confirmed that Alex Brown did break his arm during the 32-15 Heineken Cup defeat to the Ospreys.
The lock fractured the radius in his left arm and has already had surgery to have a plate inserted. He's expected to be out for six-to-eight weeks.
It is the second time Brown has undergone the same procedure after he suffered an indentical injury last year.
Gloucester head coach Dean Ryan admits it is a blow to lose such a valuable member of his squad at a crucial time in the season.
"We were pretty sure that Alex had fractured his arm on Saturday and we took every measure we could to move as quickly as possible to start his recovery process," he said.
"He is obviously a significant member of our squad and I think any side would miss such a key individual at this stage of the season.
"But we have been in this situation before and will manage. Alex is a notoriously quick healer and we will give him all the support he needs until he is fit again."