Bebe suffered a torn anterior cruciate ligament while away on international duty for Portugal Under-21s.
Bebe is currently on loan to Turkish side Besiktas after failing to make an impact at Old Trafford last season.
A Besiktas statement said: “Bebe’s injury has been reported to us and he will need to have surgery.
“He is on loan from Manchester United and both clubs will make a joint decision about where the operation will be done.”
