The striker is the club's top scorer this season after scoring 15 goals since arriving in a record-breaking deal from Peterborough United in the close-season.
However, his season was curtailed in the 67th minute of Wednesday's 3-0 victory over Wigan Athletic.
"[Assombalonga] sustained the injury while shooting against Wigan Athletic on Wednesday night," read a statement on Forest's website.
"Assombalonga underwent surgery recently and it is estimated it will take a year for him to fully recover."
The news is bad timing for Forest who have started to show signs of their early season form since Dougie Freedman replaced Stuart Pearce as manager earlier this month, winning both games the former Bolton Wanderers boss has overseen.
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