Muniain, 22, had to be stretchered off in tears during Athletic’s 2-0 defeat to Sevilla on Saturday and will now miss the Copa del Rey final against Barcelona.
“A torn cruciate ligament has been detected in his left knee,” said a statement on the club’s website after the player underwent a scan. “The estimated time out oscillates between six and seven months.”
Muniain has only one senior Spanish cap to his name, but was part of the under-21 sides that won the European Championships in 2011 and 2013.
Athletic face Barca in the Copa del Rey final for the third time in seven years on May 30.