Manchester United striker Bebe, who is currently on loan at Besiktas, has suffered cruciate ligament damage.
The 21-year-old suffered the injury while turning out for Portugal's U21 side in a fixture against Slovakia on Tuesday.
He has torn his anterior cruciate ligament and is expected to be sidelined for up to six months.
Besiktas announced the news shortly after seeing their summer recruit laid low.
The Turkish outfit claim Bebe will require surgery and they have already been in touch with United regarding a possible date.
A statement released on Besiktas' official website read: "A scan has shown an anterior cruciate ligament tear to his left knee and he will be unable to play for six months.
"Surgery is required and we have contacted Manchester United and there will be a joint decision about there the operation will be done."