The striker limped out of the goalless home draw against Doncaster last weekend with a hamstring problem and scans have revealed the damage is set to keep him out for up to a month.
Boss Billy Davies has options to replace Earnshaw, with Dele Adebola, Nathan Tyson and Lewis McGugan all available for recalls.
Loan signing Kris Boyd is also pushing for his first start after making two substitute appearances following his arrival from Middlesbrough.
Paul Anderson could come back into the team should Davies wish to make changes in midfield.
Forest will be looking to strengthen their grip on a play-off spot and reignite their automatic promotion hopes after six games without a win.