Athletic's Muniain out for six months
The club initially announced that the forward had suffered a sprained left knee during Saturday's 2-0 La Liga defeat to Sevilla.
But, following an MRI scan on Tuesday, it has now been confirmed that Muniain will be out until at least October with the problem.
Additionally, tests have revealed midfielder Ander Iturraspe has torn his adductor muscle, although Bilbao have not yet said how long his recovery will take.
Athletic are eighth in La Liga, 11 points adrift of a UEFA Europa League place.