The club confirmed he felt severe pain in his knee following the 3-1 defeat at Zenit St. Petersburg in the Europa League — and scans show there could be a serious problem.
An official statement read: "Adnan Januzaj completed the match against Zenit St. Petersburg without any complaints or apparent trauma, but an hour later began feeling intense pain in his right knee.
"The MRI scan taken yesterday showed a moderate articular effusion, which may be relation to previous meniscus operation.
"The initial plan will be to rest and undergo mild physiotherapy.
"His return to action will be progressive and will depend on how his recovery improves."