The 23-year-old Spanish international suffered ligament damage during Athletic's dramatic 4-3 victory away at Racing last weekend.
Orbaiz will have to undergo an operation on the problem, and is likely to be sidelined for at least six months.
Bilbao club doctor, Sabino Padila, confirmed that the player will not go under the knife until the end of January however while further tests are carried out.
Orbaiz himself was naturally despondent after the news, after having being cursed by injuries throughout the year.
"From a sporting point of view it has not been a good year because I have had some bad injuries," he said.
"When you go out on to the pitch you are exposed to things like this whether in the first, second or whatever division.
"But it is something that can be put right, and I'll have to try and make sure it doesn't happen again.
"It's going to be many months before I play again, but I'll return hungrier than ever."
