Lansbury suffered an ankle injury at the weekend and looks likely to be on the sidelines for the remainder of the 2015-16 campaign.
The former Arsenal midfielder was taken off on a stretcher in the second half of Forest's FA Cup third-round tie against Queens Park Rangers at the City Ground on Saturday afternoon.
The 25-year-old has revealed on Twitter that he will be out for three to four months.
Michail Antonio, who left the Reds for West Ham United in the summer of 2015, has wished Lansbury a speedy recovery on Twitter, with Forest fans responding to the midfielder’s message on the social networking site.
Not good news to update u all looking at 3/4 months out but il come back stronger— Henri Lansbury (@lansburyhenri) January 11, 2016
@lansburyhenri oh no!!!.... Gutted for you H. Stay focused and positive!!— Simon Holbrook (@SBH1978) January 11, 2016
nffc (@astonsazzle) January 11, 2016
@lansburyhenri back in time for the play offs & fa cup final then— Jordon Percival (@JordonnPercival) January 11, 2016
Lansbury has been on the books of Forest since 2012 and is one of the key figures at the City Ground.
The former England Under-21 international midfielder has scored two goals and created 52 chances in 21 Championship appearances so far this season.
The 25-year-old has a pass accuracy of 78% and a shot accuracy of 35%, and has won 17 headers, 12 headers and 42 tackles.