The striker appeared to sustain a knee injury in the 66th minute, and was not able to leave the field of his own accord, with even opposition players looking worried for him.
Assombalonga had netted in the win, scoring the opener, and Forest didn't look the same after he went off.
Manager Dougie Freedman gave an update to the club website after the game, stating that the best case scenatio at this stage appears to be that he faces a few weeks out.
He said: "It’s a difficult one. I can’t make a judgement on it right now. It doesn’t look great, but we’ll have a scan in the morning and the doctor will tell me exactly what has happened on Friday.
He’ll definitely miss the next few weeks. But we’ve got good strikers.
"Matty Fryatt came on tonight and did well, and Dexter Blackstock has been around the block and knows what’s required. Those two are more than capable of stepping in."
Whether they really are capable of matching Assombalonga remains to be seen. The £5 million summer signing from Peterborough has scored 15 goals in 27 starts this season in the Championship, and without him any hopes of reaching the play-offs become a little more distant.
Forest are are up to 10th in the division as a result of the win, but are 12 points below sixth placed Brentford.