He will now have to undergo an exploratory knee operation leaving the space for Darren Huckerby to start against Watfrod this week.
Paul Hart told the official website nottinghamforest.co.uk: "It's a major blow because nobody wants to lose a player of David's calibre for any length of time."He added: "But while we are bitterly disappointed about that, we are relieved we have got to the bottom of his problem and that he can get himself back to full fitness as quickly as possible."
Johnson has scored 24 goals this season and Forest have never lost a match when he has scored for them.