Assombalonga was stretchered off in the latter stages of Forest's 3-0 win over Wigan Athletic in midweek, and his injury has been diagnosed as a severe one.
The Congolese-born forward underwent successful surgery on his knee last night, but he will have to miss a year's worth of action - with it set to be a tough recovery process.
It is a huge blow for the player and the club as Assombalonga has been one of the Reds' key players throughout the season so far.
The 21-year-old tops the Reds' goalscoring charts and Forest fans will be hoping that this injury does not have a long-term impact on his career.
Assombalonga has the potential to make it right to the top of the game and the hope will be that this setback does not negatively impact on his development as a player.
Here is the best reaction from Forest fans to Assombalonga's injury on social media site Twitter.
Really bad news for Forest, Assombalonga out for a year, hope his recovery goes well #NFFC— John Thompson PT (@JohnThommo1981) February 13, 2015
Assombalonga out for 12 months......we have to be the unluckiest club in the world when it comes to injuries #Nffc— sam walker (@SWalker1995) February 13, 2015
#nffc is so so cursed, just when things seem to go well, our unfortunate luck takes it's toll. Feel bad for assombalonga, talented striker.— Jack Yallop (@JackYallNG) February 13, 2015
Andy Carroll may be the big-name injury today, but Forest losing Britt Assombalonga is a bigger blow. Big talent, & big future at 22. #NFFC— Jack Lusby (@jacklusby_) February 13, 2015
Gutted about Assombalonga, never thought he would be out for that long. Time for others to step up, unfortunately I dont think can #nffc— Mr Forest (@Mister_Forest) February 13, 2015
Assombalonga out for 12 monthsWorst day of the year so fara href="https://twitter.com/hashtag/nffc?src=hash">#nffc— Jason Coverleyy (@Jcov1998) February 13, 2015
Assombalonga 1 year out! Can't even sign anyone to replace. Huge blow #nffc— Ryan Whitaker (@RyanWicko) February 13, 2015
Massive blow to #nffc that Assombalonga has been ruled out for 12 months, time for other players to step up.— Niall Page (@NiallPage) February 13, 2015
Forest supporters were not the only ones to be disappointed by the news of Assombalonga's injury, either.
No football fan takes pleasure from such a talented player being on the sidelines for so long and rivalries were put to one side this afternoon with Derby County supporters also sending the striker their best wishes.
Here is the best reaction from Derby fans: